[committed 4/4] libstdc++: P1976R2 Fixed-size span construction from dynamic range

[Jonathan Wakely]

This includes fixes for first, last, as_bytes and as_writable_bytes
which were missing from the paper.

         * include/std/span (__cpp_lib_span): Update value.
         (span(It, size_type), span(It, End)): Make conditionally explicit. Add
         assertion.
         (span(R&&), span(const span<OType, OExtent>&)): Likewise and relax
         constraints.
         (span::first<Count>(), span::last<Count>()): Use explicit type in
         return statement.
         (as_bytes, as_writable_bytes): Likewise.
         * include/std/version (__cpp_lib_span): Update value.
         * testsuite/23_containers/span/1.cc: Check new value.
         * testsuite/23_containers/span/2.cc: Check new value.
         * testsuite/23_containers/span/explicit.cc: New test.


Tested powerpc64le-linux, committed to trunk.

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