Changes:

• Refactor string_util to util::string. Better to do it sooner than later, even though the string module is the only child of util right now. Signals intent that util is where all misc things that are not necessarily specific to the validator should go.
• Implement some Eq/Hash traits.
• Extend the syntax for what the validator considers to be an identifier in accordance with the ANTLR grammar and u! user-defined type prefix (the validator simply uses this to determine whether it should print something that should be a valid identifier with or without quotes in the HTML output).
• Fix some typos in comments.

Abandoned branch being my third (I think) attempt at type expression support; it was getting way too complicated the way I was doing it.