Correct EBNF grammar in the manual

The grammar for use declarations was outdated.
auto merge of #14277 : pczarn/rust/manual-grammar, r=alexcrichton

The grammar for use declarations was outdated. Corrected some other mistakes.
compiletest: Refactor compile-fail to regex.
auto merge of #14283 : Ryman/rust/commented_compile_fail_error, r=kballard

Edit: This now only covers refactoring to regex.