* Add crate arbitrary as ctap's optional dependency, when feature "fuzz"
is activated.
* Derive Arbitrary for all the necessary types in order to generate the
concrete types from random bytes.
* Add a fuzz target that transforms the input to valid format for
MakeCredential.
We don't need to build a production key without persistent storage. Tests and
fuzzing continue to use the std feature to use the RAM implementation (that does
sanity checks).
