refactor: make prove_layers_generate operate on a single partition

`prove_layers_generate()` now operates on a single partition. This makes it
clearer that Synthetic PoReps always operate on a single partition only.

With the challenge generation moved outside of this function, it's also a
clearer separation of concerns, as it now operates directly on the challenges
given, without know anything about how they were generated. This also useful
for the work of having small, special case binaries for certain operations.