Update expected-lite to proposal p0323r6 (was 5)

[martinmoene]

Update expected-lite to proposal p0323r5 p0323r6.
Provide (some) backward compatibility through CMake variable EXPECTED_P0323R.

See also issues:

• operator== Missing for void specialization, issue operator== Missing for void specialization  #29.

Plan:

1. unexpected[_type]<E>:

• constructors
• destructor (not specified)
• assignment
• swap
• observers
• relational operators: deprecate less-than comparable (provide for EXPECTED_P0323R<= 2)
• remove default template parameter std::exception_ptr (provide for EXPECTED_P0323R<= 2)
• remove unexpected[_type]<std::exception_ptr> specialisation (provide for EXPECTED_P0323R<= 2)
• alias unexpected_type&ltE> to unexpected<E> for C++17, however
  not for MSVC <= 14.1 due to clash with unexpected()

2. unexpect tag:

• struct unexpect_t
• object unexpect (now inline for C++17)
• in_place_unexpected_t kept as alias of unexpect_t
• object in_place_unexpected kept (now inline for C++17)

3. bad_expected_access:

• bad_expected_access<T>
• bad_expected_access<void>

4. expected<T,E>:

• constructors
• destructor
• assignment
• swap
• observers
• relational operators: deprecate less-than comparable (provide for EXPECTED_P0323R<= 2)
• comparison with T: deprecate less-than comparable (provide for EXPECTED_P0323R<= 2)
• comparison with unexpected[_type]<E>: deprecate less-than comparable (provide for EXPECTED_P0323R<= 2)
• specialised algorithms
• remove default template parameter std::exception_ptr (provide for EXPECTED_P0323R<= 2)

5. Traits:

• is_unexpected<> (not in propsal; provide for EXPECTED_P0323R <= 3)
• error_traits<E> (not in propsal; provide for EXPECTED_P0323R <= 3)
• expected_traits<T,E> (not present in expected-lite; removed from proposal in r4)

6. Factory functions

• make_expected, deprecate, not useful with two template argument to specify
• make_unexpected, keep

7. Test specification

• Test for all values of nsel_P0323R
• Identify missing tests
• Add missing tests

8. Update documentation

• Readme