v0.2: Extend Craystack API allowing adaptive codecs, add Multiset codec, an…

Incorporating significant changes by @dsevero:

1. Adaptive codecs
   Signatures of push and pop are extended to allow for a *context variable.

   push: (ans_state, symbol, *context) -> (ans_state, *context)
   pop: (ans_state, *context) -> (ans_state, symbol, *context)
   

   Note that, since context is passed via unpacking (i.e. *context), then it is essentially optional. Therefore, previous codecs are still compliant with this new signature.

   However, the return of push will be at least (ans_state,), so function calls have been adapted accordingly. This means that instead of

   message = codec.push(message, symbol)
   

   we now have

   message, = codec.push(message, symbol)
   

2. Multiset codec
   The multiset codec from https://github.com/facebookresearch/multiset-compression was ported to craystack/codecs/multiset.py.

3. Extra example
   An extra example, showing how to use the multiset codec for BMNIST with BB-ANS, was added to craystack/examples.