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/libstd/sync/mpsc/select.rs

@@ -61,7 +61,7 @@ use core::cell::Cell;
 use core::marker;
 use core::mem;
 use core::ptr;
-use core::uint;
+use core::usize;
 
 use sync::mpsc::{Receiver, RecvError};
 use sync::mpsc::blocking::{self, SignalToken};
@@ -228,7 +228,7 @@ impl Select {
             // A rewrite should focus on avoiding a yield loop, and for now this
             // implementation is tying us over to a more efficient "don't
             // iterate over everything every time" implementation.
-            let mut ready_id = uint::MAX;
+            let mut ready_id = usize::MAX;
             for handle in self.iter() {
                 if (*handle).packet.abort_selection() {
                     ready_id = (*handle).id;
@@ -236,7 +236,7 @@ impl Select {
             }
 
             // We must have found a ready receiver
-            assert!(ready_id != uint::MAX);
+            assert!(ready_id != usize::MAX);
             return ready_id;
         }
     }

src/libstd/sync/mpsc/shared.rs

@@ -23,7 +23,7 @@ pub use self::Failure::*;
 use core::prelude::*;
 
 use core::cmp;
-use core::int;
+use core::isize;
 
 use sync::atomic::{AtomicUsize, AtomicIsize, AtomicBool, Ordering};
 use sync::mpsc::blocking::{self, SignalToken};
@@ -33,17 +33,17 @@ use sync::mpsc::select::StartResult;
 use sync::{Mutex, MutexGuard};
 use thread::Thread;
 
-const DISCONNECTED: int = int::MIN;
-const FUDGE: int = 1024;
+const DISCONNECTED: isize = isize::MIN;
+const FUDGE: isize = 1024;
 #[cfg(test)]
-const MAX_STEALS: int = 5;
+const MAX_STEALS: isize = 5;
 #[cfg(not(test))]
-const MAX_STEALS: int = 1 << 20;
+const MAX_STEALS: isize = 1 << 20;
 
 pub struct Packet<T> {
     queue: mpsc::Queue<T>,
     cnt: AtomicIsize, // How many items are on this channel
-    steals: int, // How many times has a port received without blocking?
+    steals: isize, // How many times has a port received without blocking?
     to_wake: AtomicUsize, // SignalToken for wake up
 
     // The number of channels which are currently using this packet.

src/libstd/sync/mpsc/stream.rs

@@ -25,7 +25,7 @@ use self::Message::*;
 use core::prelude::*;
 
 use core::cmp;
-use core::int;
+use core::isize;
 use thread::Thread;
 
 use sync::atomic::{AtomicIsize, AtomicUsize, Ordering, AtomicBool};
@@ -33,11 +33,11 @@ use sync::mpsc::Receiver;
 use sync::mpsc::blocking::{self, SignalToken};
 use sync::mpsc::spsc_queue as spsc;
 
-const DISCONNECTED: int = int::MIN;
+const DISCONNECTED: isize = isize::MIN;
 #[cfg(test)]
-const MAX_STEALS: int = 5;
+const MAX_STEALS: isize = 5;
 #[cfg(not(test))]
-const MAX_STEALS: int = 1 << 20;
+const MAX_STEALS: isize = 1 << 20;
 
 pub struct Packet<T> {
     queue: spsc::Queue<Message<T>>, // internal queue for all message