Generate valid structure for MakeCredential params

* 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.

src/ctap/mod.rs

@@ -532,7 +532,7 @@ impl CtapState {
     ///
     /// This function contains the logic of `parse_command`, minus all CBOR encoding and decoding.
     /// It should make command parsing easier to test.
-    fn process_parsed_command(
+    pub fn process_parsed_command(
         &mut self,
         env: &mut impl Env,
         command: Command,