Add more Regexp, MatchData, and String APIs

This commit is in support of GH-85.

Add the following APIs:

- Regexp::escape
- Regexp::union
- Regexp#===
- Regexp#names
- Regexp#named_captures
- Regexp#options
- MatchData#length
- MatchData#named_captures
- String#scan
- String#-@

Added the constants Regexp::IGNORECASE, Regexp::EXTENDED, and
Regexp::MULTILINE.

Regexp#to_s and Regexp#inspect were erroneously implemented with the
same function. Regexp#to_s now supports roundtripping through
Regexp::compile.

Imported escape functionality from Rust regex crate.

Fix null pointer exception in String#=~ when Regexp does not match.

Reimplemented the following String APIs to work with Regexp patterns:

- String#split (TODO: does not handle blocks)
- String#sub
- String#gsub
- String#gsub!