Implemented binary expression parsing / precedence climbing.

Seems to work more or less. Needs more testing.
Implemented parsing of top-level constructs: classes, features, etc.

It's not working yet :-/ but that's probably something silly and I'll have to
debug it later - should be working really.

This also includes a change to AST code-generation, since now I really have
to handle optional nodes (specifically for Class, whose 'parent' is optional)