Hide trait rc::RcBoxPtr from docs

It is for internal use only and should not appear in docs.
auto merge of #13705 : edwardw/rust/rcboxptr-doc, r=alexcrichton

It is for internal use only and should not appear in docs.
Add regexp crate to the Rust distribution.

Also adds a regexp_macros crate, which provides natively compiled
regular expressions.

Closes #3591.

RFC: 0007-regexps