Add support from the count method from str

As a consequence, dispatch it as it is shared by the list class.
Add support for isalpha and isdigit methods from str
Harmonize comments on dispatched methods
Fix handling of Mod, Pow and FloorDiv update operator

They are not implemented the same as regular update operators.
Do not move Top-Level Class definition into main

Otherwise the SyntaxChecker detects an unknown attribute where
unsupported user classes is much more clear.
Support array concatenation through plus operator
String iterator can return char instead of char&

Python string are immutable.
Support reverse iteration on strings
Handle dummy case of an empty set temporarily updated

This result in a call to the update method from the empty_set, which is
pretty useless.