Represent doppio point in Bulletproofs

[cathieyun]

Make an AllocatedPoint type which has:

• the encoding of a doppio point (affine might be more efficient in a circuit setting, TBD)
• the Bulletproofs variables that represent the doppio point
• collection of constraints, which enforce that the point is a valid doppio point
• (maybe) a reference to a constraint system, in which it allocates the collection of contraints

[Pratyush]

Friendly note, we have an implementation of generic Twisted Edwards affine group operations in https://github.com/scipr-lab/zexe/blob/master/snark-gadgets/src/groups/curves/twisted_edwards/mod.rs that could be useful for y'all =)