Detect overflow in integer arithmetic operators (integer, smallint, and

Detect overflow in integer arithmetic operators (integer, smallint, and
bigint variants).  Clean up some inconsistencies in error message wording.
Fix scanint8 to allow trailing whitespace in INT64_MIN case.  Update
int8-exp-three-digits.out, which seems to have been ignored by the last
couple of people to modify the int8 regression test, and remove
int8-exp-three-digits-win32.out which is thereby exposed as redundant.