Minor code cleanup
Add docstrings and comments to join
Fix type checking of 'and' and 'or'

Now code such as

  s = s or ''

works as expected. Generics are also supported:

  a = a or []
Make bytes and bytearray independent types

In Python, some functions only accept bytes and some accept also
bytearrays. Previously this distinction could not be made in mypy.

Some library module stubs have not been updated to reflect this.
Add Set to builtins; add more set operations
Minor code cleanup
Code cleanup
Fix interface type member access with many bases

Previously lookup would fail for methods not defined in the first base
interface.
Fix Mapping keys/items/values to return Sets