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/librustc_resolve/lib.rs

@@ -101,7 +101,7 @@ use std::cell::{Cell, RefCell};
 use std::fmt;
 use std::mem::replace;
 use std::rc::{Rc, Weak};
-use std::uint;
+use std::usize;
 
 // NB: This module needs to be declared first so diagnostics are
 // registered before they are used.
@@ -4366,7 +4366,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         for rib in this.value_ribs.iter().rev() {
             for (&k, _) in &rib.bindings {
                 maybes.push(token::get_name(k));
-                values.push(uint::MAX);
+                values.push(usize::MAX);
             }
         }
 
@@ -4380,7 +4380,7 @@ impl<'a, 'tcx> Resolver<'a, 'tcx> {
         }
 
         if values.len() > 0 &&
-            values[smallest] != uint::MAX &&
+            values[smallest] != usize::MAX &&
             values[smallest] < name.len() + 2 &&
             values[smallest] <= max_distance &&
             name != &maybes[smallest][] {