ethsign: support for aggregate BLS signatures

BLS signature schemes are done for elliptic curves equipped suitable pairing function.
There are two immediate choices: altbn128 and BLS12-381. The former is supported on Ethereum since Byzantium while the latter is/will be used in Sapling Zcash, Eth 2.0 and Chia. I suggest we support both.

The crucial question is which library to use. I've found:
https://github.com/Project-Arda/bgls/tree/master/bgls
But not really much else...

Resources:
https://ethresear.ch/t/pragmatic-signature-aggregation-with-bls/2105
https://z.cash/blog/new-snark-curve/
https://github.com/ethereum/eth2.0-specs/blob/dev/specs/bls_signature.md#bls_aggregate_pubkeys
https://crypto.stanford.edu/~dabo/pubs/papers/BLSmultisig.html