Use libcork's 128-bit integers for IDs

Let's not roll our own.  (Or, let's roll our own in libcork in a more
general way!)  Note that this means that Pastry identifiers are now
stored in host-endian order, not big-endian as before.
Comparing pastry IDs

The `crs_id_is_cw` and `crs_id_is_ccw` functions can be used to compare
two Pastry identifiers.  Because the IDs exist on a modulo ring, the
names of the functions refer to “clockwise” and “counter-clockwise” on
that ring.  Clockwise corresponsds to >=, while counter-clockwise
corresponds to <.