Move `elliptic-curve` crate to RustCrypto/traits?

[tarcieri]

It's been pretty convenient having the elliptic-curve crate in the same repo as the curve implementations so far.

However, all of the other trait crates are presently in https://github.com/RustCrypto/traits

Aside from the consistency issue, there's a notable practical one: if we land RustCrypto/signatures#96 to reverse the relationship between the ecdsa crate and the curve implementation crates, there's now a circular dependency between crates in https://github.com/RustCrypto/elliptic-curves and the ecdsa crate in https://github.com/RustCrypto/signatures

• ecdsa depends on elliptic-curve
• ecdsa: add hazmat primitives; remove/reverse curve dependencies signatures#96 would have the k256, p256, p384 depend on both the elliptic-curve and ecdsa crates

This would make it difficult to use [patch.crates-io] directives to make incremental progress on unreleased versions of crates.

The most straightforward solution I can think of is to eliminate this circular dependency by moving elliptic-curve into the traits repo. That would allow both ecdsa and the curve implementations to use a [patch.crates-io] directive resolving to elliptic-curve in https://github.com/RustCrypto/traits

[tarcieri]

Went ahead and made this move. I think it works out for the best.