Split permutation from sponge construction

[hdevalence]

Work towards arkworks-rs/crypto-primitives#93; this doesn't touch the constraint system implementation yet,
in order to be able to get design feedback on the software part.

• The poseidon::PoseidonParameters struct is renamed to poseidon::Parameters
  but otherwise remains unchanged.

• The poseidon::PoseidonSpongeState struct is renamed to poseidon::State
  and redefined to hold just the state itself, as well as the parameters needed
  to run the permutation. It exposes a permute(&mut self) method, rate() and
  capacity() accessors, as well as Index, IndexMut, AsRef, and AsMut
  impls that allow access to the state.

• The poseidon::PoseidonSponge struct is renamed to poseidon::Sponge and
  holds a State and a DuplexSpongeMode. In other words, it consists of the
  state, together with the extra data tracking how that state is being used to
  implement a higher-level duplex construction.

• The CryptographicSponge trait is changed so that new() takes an owned,
  Self::Parameters, not a borrowed one. This allows the caller to decide
  where to copy data, instead of forcing the sponge implementation to clone
  internally. Or, a CryptographicSponge implementation could declare the
  associated Parameters type to be some shared type (like an Arc wrapper)
  that avoids the need to copy at all.

• The SpongeExt trait that allows converting back and forth between a state
  and a sponge is deleted; it's not safe to pass between abstraction layers
  that way.

---

Before we can merge this PR, please make sure that all the following items have been
checked off. If any of the checklist items are not applicable, please leave them but
write a little note why.

• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• [ ] Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
• Re-reviewed Files changed in the Github PR explorer

[hdevalence]

(A rendered copy of the docs is available here: https://rustdoc.penumbra.zone/main/ark_sponge/poseidon/index.html)

[Pratyush]

The changes look great so far, though I'm not the most qualified to review this stuff; @ValarDragon and @weikengchen are more familiar with this code. Also, is there something specific that you'd like to get feedback on?

[Pratyush]

So to clarify, these are Parameters for the underlying permutation, right? They don't have anything to do with the sponge, beyond the rate and capacity, right? If you could add a comment clarifying that, it would be great. Thanks!

[hdevalence]

Also, is there something specific that you'd like to get feedback on?

Yeah, the main thing is whether this general approach seems good, before doing the work of also updating the constraint implementations.

[hdevalence]

Hey, just bumping this -- if this approach seems good I can also apply it to the constraint implementations.

[weikengchen]

Feel free to apply to the constraints implementations.