Split up affine and projective files

[jon-chuang]

They are too long

[ValarDragon]

We're considering re-architecting the curve interfaces entirely (see #41). In that proposed API, I think it does make sense to separate out the canonical repr into its own file, where Canonical Repr is roughly what Affine Curve is rn. (Though the Canonical Repr should hopefully be smaller than it is right now)