A PublicKey is a point on Curve448.
A Secret is a Scalar on Curve448.
A safe version of the x448 function defined in RFC448. Currently, the only reason I can think of for using the raw function is FFI. Option is FFI safe. So we can still maintain that the invariant that we do not return a low order point.  https://github.com/rust-lang/nomicon/issues/59
An unchecked version of the x448 function defined in RFC448 No checks are made on the points.
A SharedSecret is a point on Curve448. This point is the result of a Diffie-Hellman key exchange.