elliptic-curve: refactor ProjectiveArithmetic trait

[tarcieri]

Renames the Arithmetic trait to ProjectiveArithmetic and changes it to have only a single associated type: ProjectivePoint.

Adds AffinePoint<C> and Scalar<C> type aliases which are able to look up the corresponding associated type chain (along with ProjectivePoint<C> for consistency, although this is available as <C as ProjectiveArithmetic>::ProjectivePoint).

[tarcieri]

Marking this as ready for review.

This was a pretty painful PR that introduced some ugly trait bounds, but I was able to remove several superfluous traits from the elliptic-curve crate (including Arithmetic, FromFieldBytes, and Generate), leaning pretty much entirely on the ones in the ff and group crates.

I've gone ahead and opened corresponding draft PRs that integrate these changes into downstream crates:

• ecdsa: refactor ProjectiveArithmetic trait signatures#164
• Refactor ProjectiveArithmetic trait elliptic-curves#179

I think this PR is a decent intermediate step towards an ff/group migration. My main concerns with it are:

• Some ugly trait bounds
• Some operations that were previously explicitly constant time aren't anymore. This mostly relates to various kinds of "repr" parsing, which is something I'd like to circle back on in terms of better integrating the ff/group serialization traits.

Regarding the ugly trait bounds, I opened one issue on ff with a small suggestion:

zkcrypto/ff#43