elliptic-curve: Use a single `Point` type instead of `AffinePoint` and `ProjectivePoint`

[carloskiki]

As discussed in #746 (comment). AffinePoint and ProjectivePoint weakly map to some elliptic curve implementations. Some implementations don't use the projective variant of their points (thinking of montgomery curves) and some only use the projective or another variant of point. For example, ed25519 uses points in $\mathbb{P}^1 \times \mathbb{P}^1$ space.

We might be able to construct a better abstraction for elliptic-curves by only using a single Point associated type. I am writing this here to stir up thoughts & opinions around this idea.

[carloskiki]

Oops, wrong repository :)