refactor(mpz-ot): ferret clean up

[sinui0]

This PR does some simplification and clean up of the ferret impl.

Changes

• Refactored to opaque error types
• Removed state from spcot and mpcot impls, making them functional
• Removed unnecessary generics
• Removed some unsatisfiable trait bounds, eg RandomCOT: Default
• Added buffering to the core types
• Implemented iterative extension with one-time bootstrapping using injected RCOT
  • This still needs work, from my understanding the iterations can be performed in parallel. Right now we're doing them sequentially and that will have poor round complexity.