This commit also does several side (related) changes. Particularly, it updates
the Eolian C generator to use the new API, it adds missing expr types (null, char)
and masks, updates the API dealing with default return values to use expressions
instead of strings and does several fixes (mainly in lexer) around the place.
It also disallows single quoted strings as those are reserved for characters.
Daniel Kolesa
Mike Blumenkrantz
Daniel Kolesa