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/libcore/ops.rs

@@ -1119,8 +1119,7 @@ impl<'a, T: ?Sized> DerefMut for &'a mut T {
 
 /// A version of the call operator that takes an immutable receiver.
 #[lang="fn"]
-#[unstable(feature = "core",
-           reason = "uncertain about variadic generics, input versus associated types")]
+#[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
 pub trait Fn<Args> {
     type Output;
@@ -1131,8 +1130,7 @@ pub trait Fn<Args> {
 
 /// A version of the call operator that takes a mutable receiver.
 #[lang="fn_mut"]
-#[unstable(feature = "core",
-           reason = "uncertain about variadic generics, input versus associated types")]
+#[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
 pub trait FnMut<Args> {
     type Output;
@@ -1143,8 +1141,7 @@ pub trait FnMut<Args> {
 
 /// A version of the call operator that takes a by-value receiver.
 #[lang="fn_once"]
-#[unstable(feature = "core",
-           reason = "uncertain about variadic generics, input versus associated types")]
+#[stable(feature = "rust1", since = "1.0.0")]
 #[rustc_paren_sugar]
 pub trait FnOnce<Args> {
     type Output;