Add span_FEATURE_WITH_CONTAINER

[martinmoene]

Add configuration flag span_FEATURE_WITH_CONTAINER to unconditionally control presence of constructors of the form span(with_container_t, ...).

Plan:

• [x] Add span_FEATURE_WITH_CONTAINER with priority above span_FEATURE_WITH_CONTAINER_TO_STD
• [x] Add span_FEATURE_WITH_CONTAINER to test/span-main.t.cpp
• [x] Document span_FEATURE_WITH_CONTAINER, mentioning it precedes over span_FEATURE_WITH_CONTAINER_TO_STD
• [x] In documentation of span_FEATURE_WITH_CONTAINER_TO_STD, mention span_FEATURE_WITH_CONTAINER takes precedence
• [x] Mention macro in table
• [x] Mention span_FEATURE_WITH_CONTAINER in test/CMakeLists.txt
• [x] Mention span_FEATURE_WITH_CONTAINER in test/t.bat, tg.bat, tc.bat

Originates from issue #32 .

Envisioned implementation:

#ifndef  span_FEATURE_WITH_CONTAINER
#ifdef   span_FEATURE_WITH_CONTAINER_TO_STD
# define span_FEATURE_WITH_CONTAINER  span_IN_STD( span_FEATURE_WITH_CONTAINER_TO_STD )
#else
# define span_FEATURE_WITH_CONTAINER  0
#endif
#endif

Originates from issue #32 .