Expand let context tree functional test to cover dependent nodes
Add unit tests for KWLetNode; expose next/previous/parent/child as readonly properties
Add unit tests to cover let node tree behaviour; fix unlinking behaviour

When unlinking a subtree of a let node tree, the parent or previous
node of the unlinked node would still have a reference to it. Inverted
the unlinking so that unlinked nodes are completely detached from the
tree.
Add test for let node working with example node visitors