Alter the API of Salmon related functions

[akihikodaki]

This commit has the following significant changes for the API

• Rename symbols with the name the specification uses.
• Introduce MagicEnvelope object in favor of reuse of parsed information and simplicity.
• Add a method to decode magic public key.

Question

I interpreted the intension of having magic_key_to_pem instead of a method just returning OpenSSL::PKey::RSA as it is expected to prevent from depending on the API of OpenSSL::PKey::RSA. However, the old OStatus2::Salmon::verify actually depends on it. Can the API of OStatus2 depend on the API of OpenSSL::PKey::RSA, or should it not?