librustc: Remove `~"string"` and `&"string"` from the language
remove leftover obsolete string literals
fix error message for obsolete &"foo" literal
auto merge of #13877 : thestinger/rust/de-tilde-str-vec, r=alexcrichton
Fix a/an typos
Add debug_assert and debug_assert_eq macros

I also switched some `assert!` calls over to `debug_assert!`.

Closes #12049.

RFC: 0015-assert
Remove useless assert! case
auto merge of #13886 : japaric/rust/fix-an-typos, r=alexcrichton

Found the first one in the rust reference docs. I was going to submit a PR with one fix, but figured I could look for more... This is the result.
auto merge of #13789 : sfackler/rust/debug-assert, r=pcwalton

I switched the `assert!` calls in `RefCell` over to `debug_assert!`.
There are probably other instances that should be converted as well, but
I couldn't think of any off the top of my head.

RFC: 0015-assert