Implements All G2 ciphersuites from the BLS Standards

[CarlBeek]

What was wrong?

0. The BLS aspects of py_ecc did not implement the interfaces specified by the BLS standards.
1. Only one of the ciphersuites was implemented. Now all three of the G2 ciphersuites are present.

How was it fixed?

By refactoring how the BLS API was implemented by building modular components that allowed for all the ciphersuites to be implemented.

Cute Animal Picture

I saw a few baby warthogs in the bush last weekend.

[CarlBeek]

I'd appreciate some assistance with the interfaces. Specifically, the way the cipher suites are implemented. Currently each ciphersuite is a file, but this leads to unused imports as certain functions needn't be redefined by the specific ciphersuite. The most obvious solution is to make each ciphersuite a folder with an __init__.py which would neaten up things at the cost of a tedious folder structure. Thoughts?

[hwwhww]

@CarlBeek
IMO it seems natural to abstract it.

1. Implement a base interface class with abc, so we will have class BaseCiphersuites(ABC) to define the interfaces.
2. Three subclass implementations in different files.

[CarlBeek]

@hwwhww, I agree classes make this much cleaner. I was avoiding them to be more zen 🧘‍♂️but it backfired.

What I don't like about what I've done now is that Sign etc are not static methods because they need access to self.DST. This means that it is necessary to instantiate the class before use which is ugly and a bad interface. Do you have a good idea on how to fix it?

[hwwhww]

LGTM! Just some nitpicking.

[CarlBeek]

This PR is ready for merge! 🎉Thereafter we can merge bls_standard into master and cut a release.