Missing constructors for SkEd25519

[jviki]

I've got sk-ed25519 public key bytes, sk application string, sk flags byte and would like to construct the PrivateKey and write it out in the OpenSSH format. I didn't find out how to do it. It seems like there are missing any constructors or means for constructing the data structs, especially KeypairData and KeyData. Do I miss something or is it currently not possible?

[tarcieri]

Note that KeypairData and KeyData are both enums with 1-tuple variants, so constructing either of them involves constructing one of the variants first, which is the real issue here.

What's currently missing are constructors for ssh_key::private::SkEd25519 and ssh_key::public::SkEd25519. I've updated the issue title accordingly.

[tarcieri]

Added in #201