major refactoring of parser and regexp

- move regexps from Hash to top-level constants
- rename and document all regexps
- remove unused regexps
- use =~ instead of .match when MatchData isn't needed
- use Regexp.match(String) form instead of String.match(Regexp)
- fully qualify all stdlib class references
- rename .nothing? to .nil_or_empty?
- add several scan optimizations in parser and reader
- slightly optimize paragraph renderer in HTML backend
- don't wrap table header cell in simpara in DocBook backend
- rename INTRINSICS to INTRINSIC_ATTRIBUTES
- fixup inconsistent syntax