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).

Partial reimplementation of compiler and TypeTable

Every type symbol and its possible polymorphic variables are now
parsed up front and put in the SymbolTable, doing away with the
complicated and error-prone NamespacedIdentifier. Still pending
changes to parts of the compiler and the runtime, so not working
at the moment.