Expose group operations

[dapplion]

trafficstars

FYI on the eth2 implementers call today, it was mentioned that we will likely want/need to expose underlying group operations in our BLS library (multiplication and addition in G2). Will be required when we add 'polynomial commitments'

https://discord.com/channels/593655374469660673/779360832588742686/784072468629422131