rustc: Fix a number of stability lint holes

There are a number of holes that the stability lint did not previously cover,
including:

* Types
* Bounds on type parameters on functions and impls
* Where clauses
* Imports
* Patterns (structs and enums)

These holes have all been fixed by overriding the `visit_path` function on the
AST visitor instead of a few specialized cases. This change also necessitated a
few stability changes:

* The `collections::fmt` module is now stable (it was already supposed to be).
* The `thread_local:👿:Key` type is now stable (it was already supposed to
  be).
* The `std::rt::{begin_unwind, begin_unwind_fmt}` functions are now stable.
  These are required via the `panic!` macro.
* The `std::old_io::stdio::{println, println_args}` functions are now stable.
  These are required by the `print!` and `println!` macros.
* The `ops::{FnOnce, FnMut, Fn}` traits are now `#[stable]`. This is required to
  make bounds with these traits stable. Note that manual implementations of
  these traits are still gated by default, this stability only allows bounds
  such as `F: FnOnce()`.

Additionally, the compiler now has special logic to ignore its own generated
`__test` module for the `--test` harness in terms of stability.

Closes #8962
Closes #16360
Closes #20327

[breaking-change]

src/libserialize/collection_impls.rs

@@ -10,7 +10,7 @@
 
 //! Implementations of serialization for structures found in libcollections
 
-use std::uint;
+use std::usize;
 use std::default::Default;
 use std::hash::{Hash, Hasher};
 use std::collections::hash_state::HashState;
@@ -148,7 +148,7 @@ impl<
     fn decode<D: Decoder>(d: &mut D) -> Result<EnumSet<T>, D::Error> {
         let bits = try!(d.read_uint());
         let mut set = EnumSet::new();
-        for bit in 0..uint::BITS {
+        for bit in 0..usize::BITS {
             if bits & (1 << bit) != 0 {
                 set.insert(CLike::from_usize(1 << bit));
             }

src/libserialize/lib.rs

@@ -31,6 +31,7 @@ Core encoding and decoding interfaces.
 #![feature(int_uint)]
 #![feature(io)]
 #![feature(path)]
+#![feature(hash)]
 #![feature(rustc_private)]
 #![feature(slicing_syntax)]
 #![feature(staged_api)]