For the sake of branch prediction, the ABI really needs to include several functions rather than the "obvious" one allocator/deallocator function to rule them all with mere convenience wrappers.

The only thing "one function" simplifies is writing interceptor libraries, and that's not sufficiently motivating (especially given that OO is a better idea). Note that it's possible for partial interception to be detected and fall back to the generic method via `if (unlikely(interposer_detected))` in every ABI-exposed implementation function.

My initial design signature for this was:

  auto utopia_alloc(Allocation, AlignAndSkew, Size, Flags) -> Allocation;

But when I actually implemented this I ended up adding an `Allocator *this` argument, then making this an inline function with fixed logic dispatching to malloc/realloc/free-like functions (but with more arguments) as needed (including a fallback if realloc isn't implemented).

Unfortunately it turns out recent versions of both GCC and Clang are gratuitously buggy regarding `malloc_usable_size` (and libcs took "advantage" of it then retroactively changed their documentation, breaking userland). Also, it turns out to be extremely useful if your libc supply a "realloc in place only" function, usually named something like "expand", but this too is not widely supported at all. Thus to make this useful I would have to implement a complete allocator, not a shim. So I kind of quit in disgust/exhaustion.