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(());

src/libstd/sys/unix/c.rs

@@ -179,20 +179,20 @@ mod select {
           target_os = "openbsd",
           target_os = "linux"))]
 mod select {
-    use uint;
+    use usize;
     use libc;
 
-    pub const FD_SETSIZE: uint = 1024;
+    pub const FD_SETSIZE: usize = 1024;
 
     #[repr(C)]
     pub struct fd_set {
         // FIXME: shouldn't this be a c_ulong?
-        fds_bits: [libc::uintptr_t; (FD_SETSIZE / uint::BITS)]
+        fds_bits: [libc::uintptr_t; (FD_SETSIZE / usize::BITS)]
     }
 
     pub fn fd_set(set: &mut fd_set, fd: i32) {
         let fd = fd as uint;
-        set.fds_bits[fd / uint::BITS] |= 1 << (fd % uint::BITS);
+        set.fds_bits[fd / usize::BITS] |= 1 << (fd % usize::BITS);
     }
 }