Add convenience methods to sign and verify w/ RSA (#2945)

This patch adds wrapper methods to allow the user to sign and verify a
single message block without having to go through the multi-step
process of creating a signer or verifier, updating it with the one
message, and finalizing the result. This will make signing and
verifying data more user-friendly when only using small messages.

Partial bug #1529
4 files changed