saner parameter order for reallocation functions

[thestinger]

Using reallocate(old_ptr, old_size, new_size, align) makes a lot more
sense than reallocate(old_ptr, new_size, align, old_size) and matches up
with the order used by existing platform APIs like mremap.

Closes #17837

[breaking-change]

[rust-highfive]

Warning

• These commits modify unsafe code. Please review it carefully!

[renato-zannon]

It might make sense to add some kind of warning to calls to reallocate for some time, because the old calls continue to typecheck, and will be silently wrong (until it fails to allocate). I only realized I had to change my code because I was randomly browsing this repo's log

[thestinger]

@renato-zannon: It's already marked as #[experimental] to indicate that it's in flux and it's probably not done changing. It's marked as [breaking-change] in the commit message too.

[renato-zannon]

Sure, I meant just as a nicety to callers whose code suddenly starts to break :p But maybe this API is too niche/low-level to be worth it, there are very few calls to it in the stdlib.

It's probably not done changing

I will take greater care then, thanks!

[thestinger]

It may take a parameter for providing the currently used size in the future. It might also allow specifying a new alignment rather than forcing it to be the same. It could also be a convenience wrapper though around functions with more parameters though... don't know at this point.