llvm_field in python

[jldodds]

It would be useful to expose something like llvm_field in the python interface.

An ideal test case would be extending this one

with another spec that uses a per-field constructor instead of the current struct operation.

Ideally we also show that we can specify only one of the structs in the record and leave the other unspecified as well.

[RyanGlScott]

I agree that we should support this, although I think that's now the domain of galois-py-toolkit rather than this repo.

[pnwamk]

Now tracking here: GaloisInc/galois-py-toolkit#4