- previous code used derived string iterators, but these would
be largely ignored anyhow since the underlying std::sub_match
str() method would just yields a std::string anyhow.
The SubStrings::str(size_t) method wasn't used in any code, so now
just use std::string iterators only.
This change simplfies overall handling, since it removes an unneeded
template dependency.
- compiler cannot decide between std::string and std::string_view
when creating from 'const char*' without also supplying the size,
so also supply a 'const char*' constructor.
ENH: additional string_view handling for ITstream and SubStrings
