Explicitly validate public key when processing Deposit

[Nashatyrev]

Deposits are the only source of non-validated public keys, so suggesting to add explicit public key validation statement to process_deposit function.

bls.Verify function (and its batch variants) is a bit ambiguous in the case if BLSPubkey is not valid (either invalid serialization or outside of the curve or outside of the group): whether it should return false or throw an error.

In all other cases (besides process_deposit) bls.Verify can't receive an invalid BLSPubkey since all of them come from the state and are inherently valid. Reasoning that way it could make sense to throw error if invalid pubkey is passed to bls.Verify since it would mean kind of fatal implementation error. In that case suggested implicit public key validation makes sense.

[dapplion]

Being explicit about this detail makes sense to me 👍

[hwwhww]

It looks fine to me, but I hope to get more reviews 👀 before we modify the phase0 spec.