Implement Go Struct Wrappers Around Proto Types

[rauljordan]

Hi all,

This issue is related to #781, as we should have Go structs as type wrappers around some of our inner proto definitions. This would make our code cleaner and align with best practices and naming conventions. For example, right now we have lines of code that look like

slashing.GetData().GetAggregateSignature_Poc1_Data().GetHash32s()

where instead we could have cleaner wrappers similar to how we defined types.Block and types.BeaconState. This is a tracking issue for any PRs that implement this functionality for some outstanding proto definitions such as ProposerSlashing, or CasperSlashing, etc.

[rauljordan]

Update: discussed offline, implementing this may not be optimal as there is no real purpose behind writing Go wrappers if we are using the Proto's as canonical types anyways. This offers no real benefit aside from better naming conventions. Additionally, this will make future refactors harder as we will need to update both the protos and the Go wrappers to have parity between the two. Closing this in favor of eventually just sticking to using protos as our types.