Fuzzer: re-serialize in C, check size

Always check that re-serializing for the sample matches the expected length of the
serializer output and that the extracted key matches the re-serialized key and its
expected serialized size.

It also includes code to check that round-tripping through the C serializer results in the
same CDR as Python, but there are is some room for variation in the encoding of mutable
types and the two do not always make the same choice. So that needs to be disabled until
they always choose the same encoding.

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