Add group/curve trait to bip-340

[cmester0]

No description provided.

[cmester0]

We could switch to using the zkcrypto/group trait, instead of using nat_mod, however it does use mutable borrows, which does not fit well with Hax.

[tarcieri]

however it does use mutable borrows

Are you talking about for the *Assign traits? Those are straight out of core::ops and all have non-*Assign variants which use immutable borrows.