std: Stablize the macros module

This commit performs a pass over the `std::macros` module, applying stability
attributes where necessary. In particular, this audits macros for patterns such
as:

* Standard use of forward-to-format-args via `$($arg:tt)*` (or `+`)
* Prevent macro-defined identifiers from leaking into expression arguments as
  hygiene is not perfectly implemented.
* Wherever possible, `$crate` is used now.

Specifically, the following actions were taken:

* The `std::macros` module itself is no longer public.
* The `panic!` macro is stable
* The `assert!` macro is stable
* The `assert_eq!` macro is stable
* The `debug_assert!` macro is stable
* The `debug_assert_eq!` macro is stable
* The `unreachable!` macro is stable after removing the extra forms to bring the
  definition in line with the `unimplemented!` macro.
* The `try!` macro is stable
* The `vec!` macro is stable

[breaking-change]

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