Move string_replace to Utils module
Add Symbol::to_string for easy printing
Support single-quote strings and underscores in integers
Lexer: Support strings in double quotes
Real numbers also support underscores
Lexer: Add other rules. Now examples/syntax.x is tokenized without errors
Set optimize and nosubs flags for the regexes:

optimize: prefers matching speed over construction of the regex object
nosubs: Doesn't return matches for the sub-patterns