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.