Should not generalise over resource declaration types

[MatthewDaggitt]

If the variable n is undeclared then

@property
robust : Vector Bool n
robust = foreach i . robustAround [[0]] 0

results in the error

Error at Line 83, Columns 1-7: The type of @property 'robust':
        >   forallT {n} . Vector Bool n
        > is not supported. Only the following types are allowed for '@property's:
        >   Bool, Vector Bool n, Tensor Bool ns
        > Fix: either change the type of 'robust' to a supported type or remove the '@property' annotation.

Not fixed by the proposed solution in #456