Implement C23 memalignment

[Wilco Dijkstra]

Hi Paul,

> I suggest adding the following paragraph, somewhat derived from Gnulib's 
> documentation for memalignment:
>
> Although this function is needed for some theoretical platforms,
> it is unnecessary on all known practical platforms.
> To check whether a pointer @var{P} has alignment @var{A},
> testing @code{(uintptr_t) @var{P} % @var{A}}
> suffices, and is typically more efficient than
> comparing @code{memalignment (@var{P})} to @var{A}.

The function definition is unnecessary just like all the abs() variants, sqrt(),
counting zeroes and so on (basically anything that is just a few instructions
and can trivially be inlined in compilers).

However we actually do want users to use higher level abstractions. And
% is a case many people still get wrong - as written above, it will actually
use a very slow division! We should never give advice that slows code
down - it's the opposite, calling the function is faster than a division...

The correct solution is to ensure compilers unconditionally inline it.

Cheers,
Wilco