librustc: Remove uses of `interner_get` in librustc
libsyntax: Remove the `interner_get` function and all uses
libsyntax: Remove an `@str` in pretty printing
librustc: Remove one more `@str` from librustc
test: Remove `@str` from the test suite
libsyntax: Remove `@str` from the interner
doc: Remove references to `@str` from the documentation
libextra: Remove `@str` from all the libraries
librustc: Remove `@str` from the language
auto merge of #11944 : nathanielherman/rust/vec_opt, r=alexcrichton

Closes #11733
Fix `@str` removal tests.
Changes from the review of the @str PR.
std,extra: remove use of & support for @[].
arena: stop using @[].
Update/delete tests using @[].
syntax: convert LitBinary from @[u8] to Rc<~[u8]>.
rustc: remove use of @[].
syntax: add an obsolete syntax warning for @[].
syntax: remove the handling of @str and @[] from the parser completely.
rustc: Remove the vstore handling of @str and @[].