Specialize span for dynamic extent (optimize static size)

[martinmoene]

From issue #3.

See also libc++ span snippet.

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...) And why did this go directly into C++20 instead of Lib Fundamentals?