Adds a copy of
string to the StringChunk.
It returns a pointer to the new copy of the string in the StringChunk. The characters in the string can be changed, if necessary, though you should not change anything after the end of the string.
Unlike insert_const, this function does not check for duplicates. Also strings added with insert will not be searched by insert_const when looking for duplicates.
the string to add
a pointer to the copy of