ECDH primitive crate

[jschneider-bensch]

This PR adds a strongly typed, owned API to the ECDH traits and establishes a "primitive" type crate for ECDH to access that uniformly across curve25519 and p256.

TODO:

• CI for the new crate
• Migrate / clean up the old libcrux-ecdh crate
• Dynamic API for selecting the curve in the primitive crate, akin to primitives/aead. (This will be a follow-up.)