Should we validate the pubkeys parameters of fast_aggregate_verify?

[ChihChengLiang]

Hi Kirk,

Currently, the fast_aggregate_verify returns true if the aggregate_public_key is an identity key because we don't run key_validate checks for the input public keys.

The IETF spec defines the behavior of FastAggregateVerify when the preconditions are met. But whether to check those preconditions are left for implementors to decide.

Here are some options:

• In Eth2's test cases, the validation is expected. It might be safer if all the client teams implement the same behavior.
• The nim-blscurve implements two versions, one with the check and one without.

[kirk-baird]

This is a good point. I think it is safe to not check these pre-conditions for fast_aggregate_verify().
This is because one of the pre-conditions is that the public key is verified by proof of possession before calling fast_aggregate_verify().

However since the check is so cheap it's beneficial to add it :)

Added in #41

[kirk-baird]

I have prevented PublicKeys from being infinity by doing subgroup checks and infinity checks during deserialisation so this should no longer be an issue.