Statically initialize constants

[robknight]

Currently calling Poseidon::new() loads the constants from scratch each time, converting them from string representation to field elements. There's ~1.8MB of constants, so this takes some time - tens of milliseconds in native and over 200ms in Wasm.

However, this process really only needs to be done once, so we can statically initialize the constants and avoid paying this cost multiple times. This greatly speeds up signing PODs in parcnet-pod:

I've also added a second commit, which looks like a huge change but isn't that big: it breaks up the constants into separate crates, enabled by features, so that it's possible to include only those constants which are required. For example, PODs only require the poseidon-1, poseidon-2, and poseidon-5 constants, and the rest are dead weight. This cuts the WASM build for parcnet-pod down from 1.7MB to 473KB. I do appreciate that this is a slightly scarier refactor, though, so we might want to discuss the implications.

[arnaucube]

This a very nice improvement!!

[arnaucube]

I tried using the lib externally, importing only the feature poseidon-2, and it breaks because the self.constants.c.len() is always 1 (same happens for the other poseidon sizes).

[arnaucube]

Maybe a way to catch this for the future, might be splitting the test into a test for each size, and adding feature conditionals to the tests; something like

    #[test]
    #[cfg(feature = "poseidon-2")]
    fn test_poseidon2() {
        // ...
    }

    #[test]
    #[cfg(feature = "poseidon-3")]
    fn test_poseidon3() {
        // ...
    }
   // ...