The old way consisted in having the parameter names, their
defaults and their annotations separated in different components
of the Arguments node. We introduced a new Param node, which holds
the name of a parameter, its default value and its annotation.
If any of the last two values are missing, then that slot will be
filled with a new node kind, Empty, which is used for specifying the
lack of something (None could have been used instead, but that means having
non-AST nodes in the Arguments node).
We're also having support for positional only arguments, for the moment
only in raw_building.

Close #215

This should prevent a StopIteration leaking when next is called
over unpack_infer.

When looking up a name in a scope, Scope.lookup will return
only the values which will be reachable after execution, as seen
in the following code:

   a = 1
   a = 2

In this case it doesn't make sense to return two values, but
only the last one.

Dict was a bit different that the corresponding class from the
builtin ast module with respect to how it was initialized.
Instead of accepting a list of pairs, the initializer accepts
two arguments, one for keys, the other for values. For backward
compatibility, the class gained a new .items property.

This means that checks such as ``isinstance(node, Lambda)`` will not
hold true anymore for Functions.
Closes #291

The mixins are better off in tree.base, rather than having their
own module. They are also used only by the AST nodes.
Close #292

do_import_module was broken for Import, since it was relying on a
missing attribute `modname`. do_import_module
in fact required all the time for a name parameter to be
passed as an argument. The inherent problem is that
Import doesn't have an underlying module name, but more of them,
in the .names attribute. Thus requiring for do_import_module to
pick one name from .names or to return multiple modules was
deemed inappropiate and the method became a function which
always requires the import name to be given.
do_import_module and real_name are now functions in interpreter.util.
Closes #293