Add differentiable binary-to-categorical module

[kevinchern]

The BinCat module/function maps a (batch of) bitstring(s) to a one-hot vector(s) and multiplies it(them) with a matrix of parameters as output. Effectively, this module turns bitstrings into categorical variables in a differentiable way.

TODOs:

• document
• test
• release note
• wrap with store_config