Creating threads

The new `cork_thread` type provides a platform-neutral API for creating
and starting threads.  See the documentation for more details.
Fix alignment in cork_array on 32-bit platforms

On 32-bit platforms, our internal representation of a cork_array (which
doesn't depend on the types of the elements) wasn't aligned correctly
for all element types.  We can grab the pointer to the internal array in
a slightly different way, however, which works for all element types
regardless of alignment.
Fix expected hashes on big-endian 32-bit systems

The expected values in our hash value test cases were incorrect on
32-bit, big-endian systems (such as PowerPC).
Merge branch 'feature/array-32bit' into develop

* feature/array-32bit:
  Fix alignment in cork_array on 32-bit platforms
Merge branch 'feature/hash-32bit' into develop

* feature/hash-32bit:
  Fix expected hashes on big-endian 32-bit systems