std: Stabilize the std::hash module

This commit aims to prepare the `std::hash` module for alpha by formalizing its
current interface whileholding off on adding `#[stable]` to the new APIs.  The
current usage with the `HashMap` and `HashSet` types is also reconciled by
separating out composable parts of the design. The primary goal of this slight
redesign is to separate the concepts of a hasher's state from a hashing
algorithm itself.

The primary change of this commit is to separate the `Hasher` trait into a
`Hasher` and a `HashState` trait. Conceptually the old `Hasher` trait was
actually just a factory for various states, but hashing had very little control
over how these states were used. Additionally the old `Hasher` trait was
actually fairly unrelated to hashing.

This commit redesigns the existing `Hasher` trait to match what the notion of a
`Hasher` normally implies with the following definition:

    trait Hasher {
        type Output;
        fn reset(&mut self);
        fn finish(&self) -> Output;
    }

This `Hasher` trait emphasizes that hashing algorithms may produce outputs other
than a `u64`, so the output type is made generic. Other than that, however, very
little is assumed about a particular hasher. It is left up to implementors to
provide specific methods or trait implementations to feed data into a hasher.

The corresponding `Hash` trait becomes:

    trait Hash<H: Hasher> {
        fn hash(&self, &mut H);
    }

The old default of `SipState` was removed from this trait as it's not something
that we're willing to stabilize until the end of time, but the type parameter is
always required to implement `Hasher`. Note that the type parameter `H` remains
on the trait to enable multidispatch for specialization of hashing for
particular hashers.

Note that `Writer` is not mentioned in either of `Hash` or `Hasher`, it is
simply used as part `derive` and the implementations for all primitive types.

With these definitions, the old `Hasher` trait is realized as a new `HashState`
trait in the `collections::hash_state` module as an unstable addition for
now. The current definition looks like:

    trait HashState {
        type Hasher: Hasher;
        fn hasher(&self) -> Hasher;
    }

The purpose of this trait is to emphasize that the one piece of functionality
for implementors is that new instances of `Hasher` can be created.  This
conceptually represents the two keys from which more instances of a
`SipHasher` can be created, and a `HashState` is what's stored in a
`HashMap`, not a `Hasher`.

Implementors of custom hash algorithms should implement the `Hasher` trait, and
only hash algorithms intended for use in hash maps need to implement or worry
about the `HashState` trait.

The entire module and `HashState` infrastructure remains `#[unstable]` due to it
being recently redesigned, but some other stability decision made for the
`std::hash` module are:

* The `Writer` trait remains `#[experimental]` as it's intended to be replaced
  with an `io::Writer` (more details soon).
* The top-level `hash` function is `#[unstable]` as it is intended to be generic
  over the hashing algorithm instead of hardwired to `SipHasher`
* The inner `sip` module is now private as its one export, `SipHasher` is
  reexported in the `hash` module.

And finally, a few changes were made to the default parameters on `HashMap`.

* The `RandomSipHasher` default type parameter was renamed to `RandomState`.
  This renaming emphasizes that it is not a hasher, but rather just state to
  generate hashers. It also moves away from the name "sip" as it may not always
  be implemented as `SipHasher`. This type lives in the
  `std::collections::hash_map` module as `#[unstable]`

* The associated `Hasher` type of `RandomState` is creatively called...
  `Hasher`! This concrete structure lives next to `RandomState` as an
  implemenation of the "default hashing algorithm" used for a `HashMap`. Under
  the hood this is currently implemented as `SipHasher`, but it draws an
  explicit interface for now and allows us to modify the implementation over
  time if necessary.

There are many breaking changes outlined above, and as a result this commit is
a:

[breaking-change]

src/libsyntax/ext/deriving/hash.rs

@@ -30,7 +30,8 @@ pub fn expand_deriving_hash<F>(cx: &mut ExtCtxt,
     let generics = LifetimeBounds {
         lifetimes: Vec::new(),
         bounds: vec!(("__S",
-                      vec!(Path::new(vec!("std", "hash", "Writer"))))),
+                      vec!(Path::new(vec!("std", "hash", "Writer")),
+                           Path::new(vec!("std", "hash", "Hasher"))))),
     };
     let args = Path::new_local("__S");
     let inline = cx.meta_word(span, InternedString::new("inline"));

src/libsyntax/ptr.rs

@@ -37,9 +37,11 @@
 //!   Moreover, a switch to, e.g. `P<'a, T>` would be easy and mostly automated.
 
 use std::fmt::{self, Show};
-use std::hash::Hash;
+use std::hash::{Hash, Hasher};
+#[cfg(stage0)] use std::hash::Writer;
 use std::ops::Deref;
 use std::ptr;
+
 use serialize::{Encodable, Decodable, Encoder, Decoder};
 
 /// An owned smart pointer.
@@ -105,7 +107,14 @@ impl<T: Show> Show for P<T> {
     }
 }
 
-impl<S, T: Hash<S>> Hash<S> for P<T> {
+#[cfg(stage0)]
+impl<S: Writer, T: Hash<S>> Hash<S> for P<T> {
+    fn hash(&self, state: &mut S) {
+        (**self).hash(state);
+    }
+}
+#[cfg(not(stage0))]
+impl<S: Hasher, T: Hash<S>> Hash<S> for P<T> {
     fn hash(&self, state: &mut S) {
         (**self).hash(state);
     }

src/libsyntax/util/interner.rs

@@ -20,6 +20,7 @@ use std::cmp::Ordering;
 use std::collections::HashMap;
 use std::fmt;
 use std::hash::Hash;
+use std::collections::hash_map::Hasher;
 use std::ops::Deref;
 use std::rc::Rc;
 
@@ -28,8 +29,8 @@ pub struct Interner<T> {
     vect: RefCell<Vec<T> >,
 }
 
-// when traits can extend traits, we should extend index<Name,T> to get .index(&FullRange)
-impl<T: Eq + Hash + Clone + 'static> Interner<T> {
+// when traits can extend traits, we should extend index<Name,T> to get []
+impl<T: Eq + Hash<Hasher> + Clone + 'static> Interner<T> {
     pub fn new() -> Interner<T> {
         Interner {
             map: RefCell::new(HashMap::new()),
@@ -78,7 +79,7 @@ impl<T: Eq + Hash + Clone + 'static> Interner<T> {
     }
 
     pub fn find<Q: ?Sized>(&self, val: &Q) -> Option<Name>
-    where Q: BorrowFrom<T> + Eq + Hash {
+    where Q: BorrowFrom<T> + Eq + Hash<Hasher> {
         let map = self.map.borrow();
         match (*map).get(val) {
             Some(v) => Some(*v),
@@ -203,7 +204,7 @@ impl StrInterner {
     }
 
     pub fn find<Q: ?Sized>(&self, val: &Q) -> Option<Name>
-    where Q: BorrowFrom<RcStr> + Eq + Hash {
+    where Q: BorrowFrom<RcStr> + Eq + Hash<Hasher> {
         match (*self.map.borrow()).get(val) {
             Some(v) => Some(*v),
             None => None,