It's currently required in src/openssl_stream.c only.

gcc and clang support __builtin_add_overflow, use it whenever
possible, falling back to our naive routines.

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.

Add some helper functions to check for overflow in a type-specific
manner.

Have the ALLOC_OVERFLOW testing macros also simply set_oom in the
case where a computation would overflow, so that callers don't
need to.

Introduce some helper macros to test integer overflow from arithmetic
and set error message appropriately.

Use an unsigned int for the version and add a helper macro so the
code is simplified (and so the error message is a common string).