The `SignatureEncoding` impl added in `rsa` v0.9 (or more specifically,
the `From<Signature>` impl for `Box<[u8]>` failed to properly left pad
the signatures so they matched the modulus size.
This adds the appropriate padding to the signature encoder.
The padding mode modules have gotten quite large.
This commit refactors types into respective submodules, with the
toplevel module defining the same-named padding schemes.
