Make Curve25519 `PublicKey`s conform to `Hashable`

Make Curve25519 PublicKeys conform to Hashable

N.B. Merge open PR #173 first (or if #173 is closed, close this one too), since this PR builds on top of that.

Checklist

[X] I've run tests to see all new and existing tests pass
[X] I've followed the code style of the rest of the project
[X] I've read the Contribution Guidelines
[X] I've updated the documentation if necessary

If you've made changes to `gyb` files

[ ] I've run .script/generate_boilerplate_files_with_gyb and included updated generated files in a commit of this pull request

Motivation:

PublicKey's are fundamental building blocks within EC-oriented code bases. They can act as a natural key in Dictionaries and otherwise important property of larger structs, which us developers often would like to make Hashable. Today, Curve25519 public keys prevent auto-synthesis of Hashable for any structs they are members of, thus Curve25519 keys ought to be Hashable!

This poses no security risk nor any performance downgrade.

This PR is an addition of open PR #173.

Modifications:

Curve25519.KeyAgreement.PublicKey and Curve25519.Signing.PublicKey have been made Hashable and unit tests have been added.

Result:

Curve25519.KeyAgreement.PublicKey and Curve25519.Signing.PublicKey are now Hashable

apple / swift-crypto