Implement method `CppStringT::join()`

[schmouk]

"str.join(iterable)

Return a string which is the concatenation of the strings in iterable. A [TypeError](https://docs.python.org/3/library/exceptions.html#TypeError) will be raised if there are any non-string values in iterable, including [bytes](https://docs.python.org/3/library/stdtypes.html#bytes) objects. The separator between elements is the string providing this method."

[schmouk]

Completed.