Adds validity checks to BLS public keys

[brunoffranca]

What ❔

Added checks for the validity of our BLS public keys as recommended by this spec. Namely we check that the public key is not zero and that it is in the correct subgroup.

We do it redundantly in two places (when we decode a public key and when we verify a signature).

Why ❔

Invalid public keys are a security risk.

[pompon0]

Please add a test for the constructor.

[dnkolegov]

Great work @brunoffranca. Thanks.

[brunoffranca]

@pompon0 I don't see a way of creating a point outside the correct subgroup. If I think of anything meanwhile, I'll add it.

[brunoffranca]

So, there was a fatal flaw in the subgroup check. To check that a given point is in the correct subgroup, we don't multiply by the cofactor, we multiply by the order. I fixed that and found a way of testing it appropriately.
Another bug is that we were not doing the validity checks for aggregate signature verification. Also fixed it.
Finally, refactored it a bit to make it more readable.