Merge #781

Merge #781

781: Add missing typechecking for enums r=philberty a=philberty

This PR splits up the Algebraic data type into one which can support many variants which is what an enum is.
It then changes the type checking for construction of ADT's to use the VariantDef structures as required. This
does not fully implement but does allow us to have most of the type checking in place to start code-generation work.

This combines work from Mark Wielaard (https://code.wildebeest.org/git/user/mjw/gccrs/commit/?h=enum-type) and Philip

Addresses #79

Co-authored-by: Mark Wielaard <mark@klomp.org>
Co-authored-by: Philip Herron <philip.herron@embecosm.com>