Check flags in point compressions

[ralexstokes]

Consider checking the flag bits in the {public key, signature} compressed representations as additional sanity checks.

From @benjaminion:

On the flags, my concern is about tooling that generates them incorrectly due to broken serialisation. It may be a miniscule risk, but I've seen crazier things. It would be ideal if the proxy would pass data if and only if it is all correct. Checking the flag against the provided y-coord is fairly simple: you just need a greater-than operation for Fps. It can be done just before the flags are masked away. To be clear for those not familiar with this: if the flag is wrong, the compressed point will deserialise to a different point than intended and the signature will fail, even if it passes the verification in the proxy contract.