McLint builtins not plugins anymore
Remove unused imports / code
Allow callbacks to be registered per message code
Clean up ReachingDefs a little
Move things around; rename natlab.toolkits.analysis.test to natlab.toolkits.analysis.core
Compute ud-du chains from reaching defs
mclint.refactoring now mclint.patterns.rewrite

The little transformations based on the unparsed patterns stuff
can't really be called refactorings.
Remove unused variables (proof of concept)
Run until fixed point, and discard obsolete messages
Naive implementation of rename variable