test-coverage-recursively: remove private and tests vocabs

private vocabs are already covered when testing the main vocab.
test vocabs don't need coverage.
tools.coverage: add tests, fixes #433
http.client: adding version of download-to that checks file exists.
spelling: use ?download-to.
extra: use ?download-to in a few more places.
sorting.quick: adding a Quicksort implementation.
sorting.quick: adding tags.txt.
stack-checker.alien: fix help-lint.
sorting.quick: faster by using stack variables not mutable locals.
io.launcher.windows.tests: many of the tests can result in utf8-encoded strings
sequences.extras: adding count* which returns % of counted items.
math.statistics: use if-zero in mean-ddof.
help.tutorial: make it more clear how the tests work, thanks @ambimorph!
sudoku: some cleanup.
sorting.quick: better interface, allow comparison to be passed in.
heaps: fix stack effect for >max-heap.
sorting.heap: adding Heapsort implementation.
math.extras: adding dotted range syntax.
math.extras: adding sum-squares and sum-cubes.
VM: refactoring that removes data_root_ranges

instead of storing data_root_ranges in data_roots, you can just store
cell pointers directly. the advantage with doing it that way is that
registration and traversal code becomes simpler (and slightly faster).