Imply span_FEATURE_MAKE_SPAN from span_FEATURE_NON_MEMBER_FIRST_LAST_SUB

[martinmoene]

Imply span_FEATURE_MAKE_SPAN when span_FEATURE_NON_MEMBER_FIRST_LAST_SUB is on, as the non-member functions first(), last() and subspan() require make_span().

Currently enabling first() etc. without enabling make_span() leads to #error.

Plan:

• In code, imply span_FEATURE_MAKE_SPAN from span_FEATURE_NON_MEMBER_FIRST_LAST_SUB
• Document span_FEATURE_MAKE_SPAN, mentioning it is implied by span_FEATURE_NON_MEMBER_FIRST_LAST_SUB
• In document of span_FEATURE_NON_MEMBER_FIRST_LAST_SUB, mention it implies span_FEATURE_MAKE_SPAN

Originates from issue #32 .