Support serialization and deserialization of ints of arbitrary length

msgpack only has built-in support for ints that fit in 64 bits. However, we
happen to be storing arbitrary json in the archive, which itself has support for
integers of arbitrary length, which themselves are mapped to "long" integers in
Python, which make the msgpack encoder blow up.

Fortunately, overflowing integers are passed to the default object hook. We
generate a msgpack "extended type" with code 1 for arbitrary integers.