Fixed insert_sstring() to work properly.
Added a start parameter to append_n_sstring().
Improved some wordings.
Added an important TODO.
Added insert_n_sstring().
Removed finished TODO - adding insert function.
Reduced the spacing between the parts of new_sstring().
Added a TODO about adding more checks.
Added 2 internat macro checks.

Mainly added to remove duplicate code and make the functions more readable.