Rename PoseidonXYZ types in poseidon module.

[hdevalence]

Summary

Rename the PoseidonParams, PoseidonSponge, PoseidonSpongeState types to Params, Sponge, SpongeState, respectively.

Problem Definition

This avoids repeating the word "poseidon" and gives the API user choice of how to name the types. In a context where it's important to specify that it's a Poseidon sponge, they can use poseidon; and refer to poseidon::Sponge; in a context where it's clear, they can simply use poseidon::Sponge; and refer to Sponge.

For Admin Use

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