This should retain most backward compatibility.

`TensorType` now supports full shape information.  The field
`TensorType.broadcastable` has been replaced by `TensorType.shape`, which
corresponds to the available static shape information.  `None`s are used to
encode unknown/no static shape information for a dimension.

These changes enforce a strict "narrowing"-only conversion policy; i.e. `Type`s
can be converted to equal or more specific types.

After fixing `Shape`'s broadcastable information,
`local_subtensor_remove_broadcastable_index` started replacing all forms of
`shape(x)[0]` with `shape(x).dimshuffle(())` when `shape(x).broadcastable
== (True,)`, so a lot of the changes in this commit compensate for that
difference.

Aside from some basic refactoring, this commit allows the
`local_useless_inc_subtensor` rewrite to handle more increment zero cases.