Add a cut-down version of std::span (array_slice)

Add a cut-down version of std::span (array_slice)

A later patch wants to be able to pass around subarray views of an
existing array.  The standard class to do that is std::span, but it's
a C++20 thing.  This patch just adds a cut-down version of it.

The intention is just to provide what's currently needed.

gcc/
	* vec.h (array_slice): New class.