Allow mutable slices in statics.

Fixes #11411
rustc: Move mut slice check to `check_static`

This is a follow-up patch that moves the mut slice check to the recently
added `check_static`

Closes #11411
Remove AtomicFlag

fixes #12943
`strdup_uniq` doesn't have to be `pub`.
rustc: disallow trailing parentheses for nullary enum variants

Fixes #12560
auto merge of #12951 : cadencemarseille/rust/issue-12943-remove-AtomicFlag, r=alexcrichton

fixes #12943
auto merge of #12957 : lkuper/rust/docsprint, r=alexcrichton
auto merge of #12742 : FlaPer87/rust/issue-11411-static-mut-slice, r=nikomatsakis

This PR enables the use of mutable slices in *mutable* static items. The work was started by @xales and I added a follow-up commit that moves the *immutable* restriction to the recently added `check_static`

Closes #11411
auto merge of #12935 : lbonn/rust/nullenum, r=alexcrichton

Fix for #12560
Docify std::vec_ng

I also removed a couple of methods that were silly and added sort.