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/sys/common/thread.rs

@@ -12,7 +12,7 @@ use core::prelude::*;
 
 use boxed::Box;
 use mem;
-use uint;
+use usize;
 use libc;
 use thunk::Thunk;
 use sys_common::stack;
@@ -25,7 +25,7 @@ use sys::{thread, stack_overflow};
 #[no_stack_check]
 pub fn start_thread(main: *mut libc::c_void) -> thread::rust_thread_return {
     unsafe {
-        stack::record_os_managed_stack_bounds(0, uint::MAX);
+        stack::record_os_managed_stack_bounds(0, usize::MAX);
         let handler = stack_overflow::Handler::new();
         let f: Box<Thunk> = mem::transmute(main);
         f.invoke(());