Asymmetry in default features: only p384 has `ecdh` by default

[hko-s]

The p384 crate differs from the other curves in that it enables the ecdh feature by default in Cargo.toml.

This is obviously not a huge deal, but I wonder if it would be good to remove it from the default features, for symmetry?

[tarcieri]

Yes, it is a bit weird it's inconsistent.

Alternatively we could add ecdh to the default features of the other crates. The only drawback is the added hkdf dependency.

[hko-s]

I'm afraid I have no useful opinion on which way to resolve this, I just stumbled over the difference :shrug:

Personally, I'm happy either way.