Repair a number of places that didn't bother to check whether PageAddItem

succeeds or not.  Revise rtree page split algorithm to take care about
making a feasible split --- ie, will the incoming tuple actually fit?
Failure to make a feasible split, combined with failure to notice the
failure, account for Jim Stone's recent bug report.  I suspect that
hash and gist indices may have the same type of bug, but at least now
we'll get error messages rather than silent failures if so.  Also clean
up rtree code to use Datum rather than char* where appropriate.