Optional but Standard SSH Signing

In the last release, RSA certificate creation via PrivateKey was introduced. In doing so a number of dependencies had to be pulled in even if you never used this feature. Thus this release makes this new functionality an optional but standard feature.

Not using the new rsa-signing features will give you 7 total dependencies from this library and trying to sign with an RSA PrivateKey will fail but in a standard build everything works normally because it would be weird for some PrivateKeys to work while others don't.

API Changes:

RSAPrivateKey

• exp: Vec<u8> --> exp: Option<Vec<u8>> and will only be Some iff the feature rsa-signing is used
• exq: Vec<u8> --> exq: Option<Vec<u8>> and will only be Some iff the feature rsa-signing is used
• The ToASN1 trait is defined iff the feature rsa-signing is used