[WIP] Open types, and type extension

[dorchard]

I think that we essentially want some types to be defined as being 'open' which means more constructors can be added freely at other points in the program, e.g., to allow:

M : UnitBase = Constructor("metres")
S : UnitBase = Constructor("seconds")

y : Float[Unit[M]] = 2.0
x : Float[Unit[M]] = 1.0
t : Float[Unit[S]] = 4.0

The intention with 'Constructor' it is a 'constructor for constructors' and can load up some fixed metadata associated with data constructors; for now I'm just designing these as strings but later this can be a dictionary.

This PR enables such definitions, extending the type environment as we go.