Possibly pointless micro-optimization for SXHASH
... type tests for CONS are more expensive than type tests for
LIST. Distinguish between CONS and LIST manually, then,
so that we can return the right answer more quickly for
... exposes a bug in the cross-compiler: SXHASH is most
definitely not constant-foldable there.
... be even more paranoid about SXHASH/PSXHASH testing