string: support insert in overload resolution

Call to insert(0, <size_type>, <CharT>) is ambiguous because literal 0
can be treated as integer or null-pointer constant.
Hence equally good candidates in overload resultion are:
- basic_string &insert(size_type index, size_type count, CharT ch)
- iterator insert(const_iterator pos, size_type count, CharT ch)
We add helper functions to resolve the ambiguity