Work towards #29; 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.

Co-authored-by: Pratyush Mishra <pratyushmishra@berkeley.edu>