Refactor the git_delta functions to have consistent naming and
parameters with the rest of the library.

Move the delta application functions into `delta.c`, next to the
similar delta creation functions.  Make the `git__delta_apply`
functions adhere to other naming and parameter style within the
library.

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.

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.

Simply switched the ordering of the checks in the for loop where this left
shift was being made.