Module mangle'

Get the mangled name of an attribute

Overloaded parameters:

:a:

    `instance`
        the instance to which the attribute belongs to. It's class name
        will be used for mangling.

    `attribute` :: string
        the attribute name to mangle

:b:
    `cls_name` :: string
        the name of the class to which the attribute belongs

Returns mangled attribute, if it starts with '__' and doesn't end with
'__'. Otherwise, `attribute` is returned :: string