Porting BLS module to py_ecc

[ChihChengLiang]

What is wrong?

As the API in BLS module stabilized, it's time to port it to py_ecc. Better benchmarking and optimization would be easier to work there.

How can it be fixed

The steps of migration:

1. Port BLSPubkey and BLSSignature types to eth_typing. The BLS module depends on these two. Add BLSPubkey and BLSSignature eth-typing#12
2. Port APIs in eth2/_utils/bls.py to py_ecc repo. Port BLS module py_ecc#51
3. On the new releases of eth_typing and py_ecc. Remove eth2/_utils/bls.py and use the functions from the packages.

[hwwhww]

Note that it becomes higher priority task because the ultra goal is wrapping BLS modules with Cython for performance! ethereum/py_ecc#50