Implement C++20 constexpr <algorithm>, , and <array>

[Jakub Jelinek]

On Fri, Mar 29, 2019 at 12:30:02PM -0400, Ed Smith-Rowland wrote:
> I see what you did there ;-) I could make my patch use the same idiom as
> what you have in the char_traits patch if folks would prefer.
> 
> BTW, what does
> 
> (void) __s;

Avoid -Wunused-parameter warning.

	Jakub