p256 crate: new high-level ECDH interface

[tarcieri]

I just cut a new release of the p256 (which it appears you're one of the only current downstream users of) which now features a high-level ECDH interface:

https://docs.rs/p256/0.4.0/p256/ecdh/index.html

It looks like it should fit your current ECDH usage patterns, but if not, please let me know!

Also I'm very interested in ways to make this implementation both work with current protocols but also be more misuse-resistant in terms of feeding ECDH shared secrets into a KDF. I'm not super familiar with how Bluetooth LE AKE works, but if that's something that interests you, please let me know!