Export parseSSHIdentity

[Sylvain Rabot]

I'm using age as a lib and I'd like to replicate the behavior of loading keys found at~/.ssh/id_(rsa|ed25519).

To do so right now I had to copy parseSSHIdentity, readPubFile and readPassphrase in my project.

It would be helpful if parseSSHIdentity was exported.

Cheers.