Should have done this ages ago, it was much easier than I expected...
this adds an explicit error clause before running the pattern match
compiler for runtime case trees, but only if the coverage checker finds
there are missing cases.
* Ignore build artifacts in 'tests' directory
* Remove unused variables in makefiles
* Add 'bootstrap-clean' rule to delete build artifacts from 'bootstrap' directory
* Add 'distclean' rule to delete all build artifacts from the source tree
