Refactor PaddingScheme into a trait

[tarcieri]

As proposed in #226, splits up the PaddingScheme enum into four structs, named after the previous variants of the struct (adopting capitalization from the Rust API guidelines):

• oaep::Oaep
• pkcs1v15::{Pkcs1v15Encrypt, Pkcs1v15Sign}
• pss::Pss

All of these are re-exported from the toplevel.

Each of these structs impls one or more of the following traits:

• PaddingScheme: used for encryption
• SignatureScheme: used for signing

The PaddingScheme constructors have been remapped as follows:

• new_oaep => Oaep::new
• new_oaep_with_label => Oaep::new_with_label
• new_oaep_with_mgf_hash => Oaep::new_with_mgf_hash
• new_oaep_with_mgf_hash_with_label => Oaep::new_with_mgf_hash_and_label
• new_pkcs1v15_encrypt => Pkcs1v15Encrypt
• new_pkcs1v15_sign => Pkcs1v15Sign::new
• new_pkcs1v15_sign_raw => Pkcs1v15Sign::new_raw
• new_pss => Pss::{new, new_blinded}
• new_pss_with_salt => Pss::{new_with_salt new_blinded_with_salt}

[tarcieri]

cc @lumag

[tarcieri]

Note that there's various additional refactoring that could probably be performed, like getting rid of the inherent methods for these functionality and using the traits exclusively, or getting rid of the SignatureScheme trait and factoring the relevant code into SigningKey and VerifyingKey.

I deliberately didn't make any of those kinds of changes to make code review easier.

[dignifiedquire]

I overall very much like this approach, it makes the API clearer to understand and reason about.

[tarcieri]

@dignifiedquire this should be good to go now

[dignifiedquire]

thank you, lgtm