I added this originally because it's part of Python's grammar, but since
we merged parenthesis and friends into expressions instead of forming
separate nodes, BaseAtom doesn't have much use.
This fixes also a few bugs:
- A decorator requires a `dotted_name`, so it can only take a `Name` or
an `Attribute` node, not any atom.
- A `Call` node's function can be any expression, not just a fixed list
of `BaseExpression` subclasses.
- Now that we're exporting everything from the top-level libcst package,
automodule makes a mess of things.
- While it does create more boilerplate, using autoclass/autofunction
instead of automodule gives us a lot more control over the order in
which nodes are presented, so we can group them by various categories.
This diff also exports BaseParenthesizableWhitespace, because nodes
refer to it by type, and so it's needed for documentation purposes.
Without this option, autodoc defaults to ordering all members of
modules and classes alphabetically.
While this is normally fine (and probably desirable), the ordering of
fields in dataclasses is significant, so we need to change this behavior
to preserve the original member ordering.
