This extracts the `difficulty` module from `block` and the
`equihash_solution` module from the crate root. The PoW calculations
are significantly more complicated than the other block code and pretty
dissimilar from it, so it makes more sense to create a common proof of
work module.
The `EquihashSolution` and `EQUIHASH_SOLUTION_SIZE` are renamed to
`equihash::Solution` and `equihash::SOLUTION_SIZE` and imported that
way, except in `block/header.rs`, to avoid a conflict with the
`equihash` crate. In the future it would be better to encapsulate the
equihash solution check into the `equihash::Solution` type so that
callers only need to import our `work::equihash`.
The test organization leaves a little to be desired but I think that
this can be improved as we fill out the proof of work implementation.
Henry de Valence