ssh_key::Signature doesn't round-trip through Signature trait

[programmerjake]

Afaict the Signature::from_bytes method expects a algorithm type string as a prefix, but Signature::as_bytes doesn't return the algorithm type string as a prefix.

[tarcieri]

To make this work the internal data field would need to be changed to store the entire serialized signature, including the algorithm prefix, with a separate helper method to access the signature data sans prefix.

[tarcieri]

I've opened a PR for a prospective signature v2.0 which separates a signature type from its serialization:

RustCrypto/traits#1141

Rather than fix this issue for the signature v1.0 API, I think it would be best to hold off on changes until the new signature API is available, where this issue will effectively be automatically addressed by the new API.

[tarcieri]

This should be addressed by #55, namely the changes here:

https://github.com/RustCrypto/SSH/pull/55/files#diff-a7d9c3005184f746fc7019de8c392d8edda5186a4c0e5e07ced7d152b0bc75d9