Implement method `CppStringT::maketrans()`

[schmouk]

"static str.maketrans(x[, y[, z]])

This static method returns a translation table usable for [str.translate()](https://docs.python.org/3/library/stdtypes.html#str.translate).

If there is only one argument, it must be a dictionary mapping Unicode ordinals (integers) or characters (strings of length 1) to Unicode ordinals, strings (of arbitrary lengths) or None. Character keys will then be converted to ordinals.

If there are two arguments, they must be strings of equal length, and in the resulting dictionary, each character in x will be mapped to the character at the same position in y. If there is a third argument, it must be a string, whose characters will be mapped to None in the result."

[schmouk]

Implementation should be done via std::map and union types, one for keys and another one for values.

[schmouk]

Finally implement as an internal class named TransTable.