Review span in face of p1024 and Tristan Brindle's span implementation

[martinmoene]

References:

• p1024
• Tristan Brindle's Implementation of C++20's std::span for older compilers..

Plan:

• [x] Add front() and back() member functions (p1024)
• [x] Mark empty() as [[nodiscard]] (p1024)
• [x] Add (macro for) deprecation messages
• [x] Remove/deprecate operator() for element access (p1024)
• [x] Rename namespace details to detail.
• [x] Rename span_CONFIG_PROVIDE... to span_FEATURE....
• [x] Rename span_CONFIG_CONTRACT_CHECK() to span_CONTRACT_CHECK().
• [x] Rename to span_FEATURE_MEMBER_BACK_FRONT, span_FEATURE_MEMBER_SWAP.
• [x] Make sure span_HAVE_* are defined.
• [x] Name exception contract_violation, function report_contract_violation(....): throw, or terminate.
• [x] Fix span_CONFIG_CONTRACT_CHECK for contract-violation-terminates, issue #15.
• [x] span_constexpr_exp: constexpr (elided), or span_constexpr14.
• [x] span_constexpr_ens: constexpr (elided), or span_constexpr14.
• [x] Add contract non-empty to front(), back()
• [x] Add member at(index_type).
• [x] Add non-member versions of first(), last() and subspan().
• [x] Allow for index_type to be unsigned/size_t (p1089), see #21

Deferred to issue #30:

• Use feature test macros: defined(__cpp_deduction_guides) etc.
• Differentiate storage for dynamic, static extent.
• Structured bindings support for fixed-size spans (only non-dynamically-sized spans) (p1024)
• Allow also for Extent to be unsigned with dynamic_extent numeric_limits<index_type>::max() (p1089)?