Update BLS signature module

[hwwhww]

What is wrong?

Update it with the latest spec.

How can it be fixed

Currently, the module is here: https://github.com/ethereum/py-evm/blob/master/eth/utils/bls.py

1. Replace BN128 with BLS-12-381 curve.
2. Implement and BLSVerify and BLSMultiVerify from spec. Pseudocode:

   def multi_verify(pubs, msgs, sig):
       len_msgs = len(msgs)
       assert len(pubs) == len_msgs
       o = FQ12(1)
       for m in set(msgs):
           group_pub = G1.zero()
           for i in range(len_msgs):
               if msgs[i] == m:
                   group_pub = add(group_pub, pubs[i])
           o *= pairing(group_pub, m, final_exponentiate=False)
       o *= pairing(neg(G1), sig, final_exponentiate=False)
       return o ** ((field_modulus ** 12 - 1) // curve_order) == 1

3. Plus aggregate_sigs, pseudocode:

   def aggregate_sigs(sigs):
       o = FQ12(1)
       for s in sigs:
           o *= s
       return o

4. Replace the current blake hashing function with keccak-256.

[hwwhww]

closing via #1581