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).

Put expression types of procedures in type table

Before we annotated the AST. But that is silly in hindsight:
certain operations have a completely different meaning when applied
in a polymorphic context (i.e. the field index determined by a
select expression).