Support compositions in the forge ffi prove function

[nategraf]

Forge FFI is used in the tests to call out to external programs, and in our testing code is used to generate receipts that can then be used to test receipt-authorized logic. This functionality is also referred to as cheat codes. The prove cheat code used in tests supports only a subset of the prover API, and one particularly useful feature that we would like to support is providing assumption receipts for composition.