Python serializing of keys in mutable types

This addresses two issues:

Firstly, a keyless type that is a use as a key field needs to be treated as-if all fields
are key fields.  It wasn't serializing all of these.

Secondly, the serializer in the Cyclone core sets the must-understand bit on fields
actually used as key fields (not all implementations make this choice, the spec is
ambiguous), but the Python one did not consistently do so: in a non-key field of an inner
struct itself containing fields marked as keys, and in a key field of an inner type struct
containing no key fields it would deviate.  That leads to fuzzing failures.

Signed-off-by: Erik Boasson <eb@ilities.com>