libstdc++: Add pretty printer for std::span

This improves the debug output for C++20 spans.

Before:
{static extent = 18446744073709551615, _M_ptr = 0x7fffffffb9a8, _M_extent = {_M_extent_value = 2}}
Now with StdSpanPrinter:
std::span of length 2 = {1, 2}

Signed-off-by: Philipp Fent <fent@in.tum.de>

libstdc++-v3/ChangeLog:

	* python/libstdcxx/v6/printers.py (StdSpanPrinter): Define.
	* testsuite/libstdc++-prettyprinters/cxx20.cc: Test it.