Write generators for arbitrary ill-typed programs

[DavidGregory084]

At the moment we have generators for arbitrary well-typed monomorphic programs. This enables us to test to a limited degree that well-typed programs can be compiled successfully, but it does not provide any reassurance that ill-typed programs will fail compilation.

The only way that I have seen others do this is to generate well-typed programs and then make some small change to the program which breaks one of the inference rules for the type system. This is likely to be very tricky!