Expose an API for calculating catcache hash values.

Now that cache invalidation callbacks get only a hash value, and not a
tuple TID (per commits 632ae682 and
b5282aa8), the only way they can restrict
what they invalidate is to know what the hash values mean.  setrefs.c was
doing this via a hard-wired assumption but that seems pretty grotty, and
it'll only get worse as more cases come up.  So let's expose a calculation
function that takes the same parameters as SearchSysCache.  Per complaint
from Marko Kreen.