Algebraic Sponge Algorithms

[bhgomes]

@SupremoUGH: Strange trait. Poseidon block elements are field elements, which you can add and subtract already. Why not import a suitable trait from algebra?

@GhostOfGauss: Not sure what trait we'd use from algebra, I don't see anything with a sub function.

@bhgomes: This concept of BlockElement is a general construction for all "algebraic" duplex sponge algorithms. We don't explicitly use any of the poseidon-specific constructions here and this is the only "algebraic" concept we need. This file should eventually be factored out into something like an algebraic sponge module.

Originally posted by @SupremoUGH in #34 (comment)