Make various methods private

[drhagen]

Partially implements #95.

• Rename StringReader.current_line to _current_line
• Extract handle_other to global function
  • Rename handle_other to wrap_literal
  • Use wrap_literal in all parsers
  • PredicateParser was not wrapping literals at all, now is
  • Most parsers only wrapped str, now all wrap non-Parsers
• Rename consume to _consume
• Rename cached_consume to consume