The occam parser is now a GenParser Token OccState, rather than a GenParser
Char OccState, and a lot of now-redundant code has been removed. The parser is
also somewhat faster, which wasn't intended but is nice anyway.
I've also modified the Rain parser to not rely on the old preprocessing code;
it wasn't appropriate for Rain's syntax anyway, so I assume Neil will be
replacing it eventually.
The parser now gets a stream of tokens, rather than needing to worry about
loading files itself.
This also reworks the lexer's idea of what constitutes a Token -- it's now a
pair (Meta, TokenType), so it's always easy to pull out/rewrite the metadata --
and adds proper support for lexing preprocessor directives, rather than just
treating them as reserved words.
