pkcs8: impl `FromPrivateKey`/`FromPublicKey` on respective document types

[tarcieri]

This issue proposes, as a breaking change in the next release, to impl the following:

• FromPrivateKey (and potentially ToPrivateKey) on PrivateKeyDocument
• FromPublicKey (and potentially ToPublicKey) on PublicKeyDocument

The methods in these traits both duplicate and thunk through the methods on the respective document types, so impl'ing the traits would consolidate all of the functionality in one place.

Another additional benefit is that methods like EncryptedPrivateKeyInfo::decrypt could be made generic around a T: FromPrivateKey return type, making it possible to decrypt encrypted private keys to any type which impls FromPrivateKey.

Likewise an encrypt method could be added to ToPrivateKey.

[tarcieri]

Fixed in #110