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/librustdoc/passes.rs

@@ -12,7 +12,7 @@ use std::collections::HashSet;
 use rustc::util::nodemap::NodeSet;
 use std::cmp;
 use std::string::String;
-use std::uint;
+use std::usize;
 use syntax::ast;
 use syntax::ast_util;
 
@@ -310,7 +310,7 @@ pub fn unindent(s: &str) -> String {
     let lines = s.lines_any().collect::<Vec<&str> >();
     let mut saw_first_line = false;
     let mut saw_second_line = false;
-    let min_indent = lines.iter().fold(uint::MAX, |min_indent, line| {
+    let min_indent = lines.iter().fold(usize::MAX, |min_indent, line| {
 
         // After we see the first non-whitespace line, look at
         // the line we have. If it is not whitespace, and therefore
@@ -322,7 +322,7 @@ pub fn unindent(s: &str) -> String {
             !line.chars().all(|c| c.is_whitespace());
 
         let min_indent = if ignore_previous_indents {
-            uint::MAX
+            usize::MAX
         } else {
             min_indent
         };