When looking at how this would generlaize to the other public key
cryptosystems (ECDSA, ED25519, etc.), I think having fewer submodules
involved makes more sense.
Take a step towards flattening (and simplifying) the public API of
the RSA submodule. This is done as a separate step from the rest of
the work so that the Git history will correctly reflect that signing.rs
gets renamed to keypair.rs, with only minimial modifications, in the
next commit. (If this were merged with the following commit, then Git
would report the new keypair.rs as a new file without any history from
signing.rs.)
