Use cork_u128 in big hash values

Hey look, big hash values are just 128-bit integers.  Let's use our new
`cork_u128` type for those!
Other hash types in cork-hash

The cork-hash utility now takes in a handful of command-line options
that let you choose between the different kinds of hash value that we
can produce: stable (the default, reproducing the previous behavior),
fastest, and big.  Just like the underlying C functions, the final two
are not guaranteed to provide consistent results on different platforms,
or across different versions of libcork.