Add PEM/DER APIs for Curve25519

[ptoffy]

Add PEM/DER APIs for Curve25519

Checklist

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

If you've made changes to gyb files

• I've run ./scripts/generate_boilerplate_files_with_gyb.sh and included updated generated files in a commit of this pull request

Motivation:

Closes #82
Creating Ed25519/X25519 keys using PEM/DER and also generating their PEM/DER representations can be handy

Modifications:

Added

• init(pemRepresentation:) and the pemRepresentation computed property
• init(derRepresentation:) and the derRepresentation computed property

to Curve25519.{Signing,KeyAgreement}.{Private,Public}Key

Result:

Curve25519 can now be created with and can produce PEM/DER representations

[Lukasa]

Can we also add derRepresentation properties to all of these? It's rarely useful to add one without the other.