Initial casting implementation

Explicit casts can be performed with the syntax 'cast<type>(input)'
and implicit casts can be performed with the syntax 'cast(input)'
where the output type is determined by inference.

To prevent casting shenanigans we only allow casting of primitive
types and of types to themselves (essentially creating a copy).

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.