Adds a copy of
string to the StringChunk, unless the same string has
already been added to the StringChunk with insert_const.
This function is useful if you need to copy a large number of strings but do not want to waste space storing duplicates. But you must remember that there may be several pointers to the same string, and so any changes made to the strings should be done very carefully.
Note that insert_const will not return a pointer to a string added with insert, even if they do match.
the string to add
a pointer to the new or existing copy of