Remove dead code.
Add a 'check' target for better testing.

This makes CTest output the results of failed tests, by default.
Add new clang-format for new code.

New code does not have to use the same formatting as existing code.
Add core/data directory and initial file.

tree.hpp is a straight copy of src/Tree.h and will serve the same
purpose, though with testing and a cleaner api.
Use proper include guard.
Remove dependency on Qt.
Add test and update build.
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