Rewrote typing to use indices.

Currently it is slower than before, because we do a HashMap lookup
followed up by actually using the index. But it serves as the basis
for a faster type inferencer.

The main goal, however, is to fix the manner in which polymorph
types are determined. The typing queue of functions still needs to
correctly write this data to the type table.

Initial union implementation

Still untested, but: make union declaration implicit (so no longer
inferred from 'enum' use), and initial implementation of type
inference.