Add hal::set_exception_allocator(Callable&)

[kammce]

Such a callable may have a signature such as this:

void* memory_handler(size_t p_object_size, bool p_allocate);

Bool parameters are not advised but this sells the point. If allocating, the void* is returned out from __cxa_allocate_exception. If deallocating, then the return value is ignored. Allocating exception memory should set the data to all zeros before returning it. Memory CAN be memset to zero for deallocating the memory, but isn't required. This is the base idea. This can change as implementation begins.

[kammce]

I don't remember the rationale or motivation for this. I think using PMR is the right choice even though its a bit annoying to deal with the fact that PMR provides you the size as well.

[kammce]

Replaced by #20