Make our overflow check look more like gcc/clang's
Make our overflow checking look more like gcc and clang's, so that
we can substitute it out with the compiler instrinsics on platforms
that support it. This means dropping the ability to pass `NULL` as
an out parameter.
As a result, the macros also get updated to reflect this as well.
allocations: test for overflow of requested size
Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.