Backwards-compatibility of imports broken after refactoring

[matjobst]

After the refactoring #78 , the code structure of NIR changed.

Many libraries were using imports such as

from nir import NIRNode

However this fails now as NIRNode is not visible anymore. The solution should be to add it to the list of imports in the __init__.py that looks something like this:

from nir.node import NIRNode
...
__all__ = ["NIRNode", ...]

This would allow the simpler imports again.

[matjobst]

Actually the NIRNode just seems to be missing in __all__ and __all_ir of nir/ir/__init__.py