RSA module specifics

[WingZer0o]

The // TODO: do a check for key_size is still open — sizes < 2048 should be rejected.
:52 if verified.is_err() == false { return true } else { return false } is just verified.is_ok().
Pkcs1v15Sign::new_unprefixed() for signing is a footgun — consider exposing PSS, and OAEP for encryption, rather than raw PKCS#1 v1.5.