Complete the implementation of stakeDistr

[carlostome]

As per @WhatisRT comment:

Ah, this function was only partially implemented. A correct implementation would need PState, so I don't think it's a good idea to remove it.

If you want to finish the implementation, look at Fig. 37 of the Shelley spec. Note that addrPtr has now been deprecated and removed, so that part is intentionally missing. This is also one of the few places where the spec really leans into set theory, so the inverse here turns a function into a relation and performs some operations on relations. I think stakeRelation is correct in this code.

Originally posted by @WhatisRT in #670 (review)