As the LexicalEnv.get method always return an array of root nodes
while the root node is defined in user code, we need to turn the Node
and LexicalEnv classes into generic traits and instantiate them using
the actual root node.
To make this works, root node declaration has to be changed from
`class FooNode : Node` to `class FooNode implements Node[FooNode]`,
which required to refactor a bit almost all the lkt tests.
- Allow just subclassing App.process_unit, by providing a default main
that iterates on units and calls process_unit on each
- Allow passing args to python App programmatically
