[WIP] p256 acceleration

[austinabell]

Currently accelerates the muls and is ~8-9x speedup, but there is a lot of slop due to the unaccelerated conversion from projective to affine. Also unfortunate that this implementation has BE representation internally, so some unfortunate copies again as with k256.

TODO:

• Accelerate proj -> affine -> proj
• Reduce conversion logic
  • Unfortunate this is so generic, and there might be some extra copies that can't be guaranteed to be optimized away
• Explore different bit widths acceleration
  • Currently just errors in compile time with this (which I think is correct, shouldn't accidentally be using this for other crates in this repo
• Add acceleration for other code paths other than just mul (not sure where/how it might be used)
• Check const time signature gen (or panic on usage in zkvm)