Make it so ASDF 3.3 and LIFT won't complain.

Fix concurrency issue in red-black-tree

The old implementation used a global *RBT-EMPTY-NODE* for all instanced
of red-black-tree. This caused an error when multiple threads were
updating different trees at the same time.

Further discussion here: https://github.com/gwkkwg/cl-containers/issues/6

Fix the cl-containers-test.asd file to remove a non-existent static file.

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The one chunk of code in here (not active) got fixed up and moved to the other trees test file.

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When searching for an item in a bst, make sure that the node isn't a placeholder and actually has an element

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