Cached group checks

[mratsim]

This uses the new primitives mentioned in #92 for cached group checks.

Now we check at deserialization:

• if a pubkey is infinity or not in the subgroup
• if a signature is not in the subgroup (we may deserialize empty signatures)

Instead of checking that at verification time. Since public keys are reused multiple time this is a significant performance improvement.

New perf

In #97 we had

So we cumulated a total of (4025212-4588537)/(4588537) = 12.2% perf improvement compared to the current BLST in Nimbus.

Closes #42 as well

[mratsim]

okay so I broke the PR ... I didn't close it ... nor did I merge it