Modify the ``EnumType.__dir__()`` and ``Enum.__dir__()`` to ensure
that user-defined methods and methods inherited from mixin classes always
show up in the output of `help()`. This change also makes it easier for
IDEs to provide auto-completion.
* Enum: streamline repr() and str(); improve docs
- repr() is now ``enum_class.member_name``
- stdlib global enums are ``module_name.member_name``
- str() is now ``member_name``
- add HOW-TO section for ``Enum``
- change main documentation to be an API reference