added get_mut() for [T]
auto merge of #14750 : bachm/rust/master, r=alexcrichton

This adds the missing `get_mut` method to the `MutableVector` trait, and implements it for `&'a mut [T]`.
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rustc: Obsolete the `@` syntax entirely

This removes all remnants of `@` pointers from rustc. Additionally, this removes
the `GC` structure from the prelude as it seems odd exporting an experimental
type in the prelude by default.

Closes #14193
[breaking-change]