Support for single-quoted strings

- token class handles both single and double quoted strings. Single quoted
  strings are used to tag regular expressions. At the moment this is just
  syntactical sugar and isn't (yet) treated differently than double-quoted
  strings.

- write output for std:string, with/without single quotes with the method
  writeQuoted(). Use distinct method name to avoid inadvertent compiler
  conversions.

- write wordRe and keyType using writeQuoted()