The GIN code has absolutely no business exporting GIN-specific functions
with names as generic as compareItemPointers() or newScanKey(); that's
just trouble waiting to happen.  I got annoyed about this again just now
and decided to fix it.  This commit ensures that all global symbols
defined in access/gin/ have names including "gin" or "Gin".  There were a
couple of cases, like names involving "PostingItem", where arguably the
names were already sufficiently nongeneric; but I figured as long as I was
risking creating merge problems for unapplied GIN patches I might as well
impose a uniform policy.

I didn't touch any static symbol names.  There might be some places
where it'd be appropriate to rename some static functions to match
siblings that are exported, but I'll leave that for another time.