Add more Regexp, MatchData, and String APIs

[lopopolo]

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!