Review span in face of p1024, p1089, p1872 (Continued from #14)

[martinmoene]

References:

• Continued from #14
• p1024 - Usability Enhancements for std::span,
  voted in, 2019-02 Kona ISO C++ Committee Trip Report
• p1085 - Should Span be Regular?: No comparison operators,
  voted in, 2018 Rapperswil ISO C++ Committee Trip Report
• p1089 - Sizes Should Only span Unsigned - see #27
• p1872 - span should have size_type, not index_type,
  voted in, 2019-11 Belfast ISO C++ Committee Trip Report

Plan:

• Rename as_writeable_bytes() to as_writable_bytes() (in r7, thanks to @nolange, see below)
• Structured bindings support for fixed-size spans (only non-dynamically-sized spans) (p1024)
• Change default of span_FEATURE_COMPARISON to 0 (p1085)
• Change default of span_CONFIG_SIZE_TYPE to std::size_t (p1089)
• Change default of span_CONFIG_EXTENT_TYPE to std::size_t (p1089)
• Allow also for Extent to be unsigned with dynamic_extent numeric_limits<index_type>::max() or static_cast<extent_t>(-1); (p1089)
• Rename index_type to size_type (p1872)

---

• Differentiate storage for dynamic, static extent: moved to #8 (duplicate #47)
• Use feature test macros: defined(__cpp_deduction_guides) etc.: moved to #48

[nolange]

some small change was apparently missed:
rename as_writeable_bytes -> as_writable_bytes