Change float8-to-int8 conversion to round to nearest, rather than

Change float8-to-int8 conversion to round to nearest, rather than
truncating to integer.  Remove regress test that checks whether
4567890123456789 can be converted to float without loss; since that's
52 bits, it's on the hairy edge of failing with IEEE float8s, and indeed
rint seems to give platform-dependent results for it.