Honor hidden doc attribute of derivable trait methods

Closes #13698
auto merge of #13704 : edwardw/rust/doc-hidden, r=alexcrichton

Closes #13698
Add a regex crate to the Rust distribution.

Also adds a regex_macros crate, which provides natively compiled
regular expressions with a syntax extension.

Closes #3591.

RFC: 0007-regexps