Replace uses of empty type application by references

[andrevidela]

Currently, we use empty type application to represent in-scope references. This poses some problems when resolving specialisation references in the extendContext and inside each backend.

A solution to this would be to incrementally build the context from the list of previous definitions:

(name bit (+ 1 1))
(name byte (* bit bit bit bit bit bit bit bit))

would be represented as:

Vect 2 (TDef' 0 True)

and the first step of compilation would map this structure to

(context: Vect 1 (TDef' 0 False), remaining: Vect 1 (TDef' 0 True))

Just like a typical compiler would add variable declaration to the context.

[andrevidela]

Basically stop using TApp"type" T0 [] for references that are replaced last minute and pull them from an incrementally built context