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/lib.rs

@@ -164,6 +164,7 @@ pub use core::cell;
 pub use core::clone;
 #[cfg(not(test))] pub use core::cmp;
 pub use core::default;
+#[allow(deprecated)]
 pub use core::finally;
 pub use core::hash;
 pub use core::intrinsics;
@@ -306,8 +307,8 @@ mod std {
     pub use marker;  // used for tls!
     pub use ops; // used for bitflags!
 
-    // The test runner calls ::std::os::args() but really wants realstd
-    #[cfg(test)] pub use realstd::os as os;
+    // The test runner calls ::std::env::args() but really wants realstd
+    #[cfg(test)] pub use realstd::env as env;
     // The test runner requires std::slice::Vector, so re-export std::slice just for it.
     //
     // It is also used in vec![]