Cache public key group membership status

[benjaminion]

PR Description

Group membership checks for elliptic curve points (public keys and signatures) are very expensive and we want to do them as little as possible, ideally checking each point we use in verification exactly once.

When aggregating public keys using Blst's blst.P1.aggregate() method, the keys' group membership is always checked, even if it was checked previously. With this PR, we lazily check a public key's group membership and cache the result. We can then avoid the repeated group membership checks by using blst.P1.add() instead of aggregate().

Remarks:

• This only helps if BLSPublicKey objects are well-cached and not created anew: we lose the cached group check status in that case.
• By design, the aggregated public key will be in the group. Nevertheless Blst's core_verify() still checks group membership of the aggregated key. Not a lot we can do about that.
• As for signatures, we don't expect to be seeing the same signature time and time again, so there is likely little to be gained by applying the group membership caching to signatures.

Fixed Issue(s)

Closes #4032

Documentation

• I thought about documentation and added the documentation label to this PR if updates are required.

Changelog

• I thought about adding a changelog entry, and added one if I deemed necessary.

[ajsutton]

LGTM.