Re: No variables available for parameterised types

[Chris Warburton]

Added the "''Enumerable" line to MLSpec's theory renderer (qualified,
along with the necessary module imports and package dependencies for
testing-feat).

We're now getting variables of parameterised types like lists.

We're still not getting arbitrary function variables, e.g. for use in
map, fold, etc. even though we're asking for them.

Looking at the default instances of Arbitrary, it looks like we need to
satisfy a CoArbitrary constraint too, which ifcxt may be missing since
we're not giving it access to CoArbitrary.