WIP: Busy with MPSC channel

Will act as backing buffer for messages (for now).

Add a new RawArray implementation

Needed to implement ringbuffer queues in a neat way. Will deprecate the
old RawVec implementation

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).