Specialize span for dynamic extent (optimize static size)

From issue #3.

See also [libc++ span snippet](https://github.com/mclow/snippets/blob/master/span.cpp).

Note remark by Nevin Liber in [isocpp-lib] Another span question, on 24-Apr-18 22:55:
>Meta-questions:  why are we conflating fixed extents and dynamic extents into the same class template?  (shades of vector<bool>...)  And why did this go directly into C++20 instead of Lib Fundamentals?