Fix typos in tutorial

parrent > parent, url > URL
Requirements made slightly more complete: cf https://github.com/ianmiell/shutit/blob/master/library/jq/jq.py
Print stack value refcounts when tracing (#636)
Enable printing of stack val refcnts
STOREV/LOADV* should also print refcnts
Fix refcount leak, fix #618
fix typo in manual
Merge pull request #642 from Janrain-Colin/patch-3

fix typo in manual
Add --argjson, fix #648
Fix #647, but caused by 89791a0
Add until(cond; next); fix #639
Merge pull request #616 from ianmiell/ianmiell_requirements

Requirements made slightly more complete
Merge pull request #611 from StevenMaude/tutorial-typo-fixes

Fix typos in tutorial
Implement flatten\0 in terms of flatten\1