Treat a fieldless exists leaf as match-all and make the lenient set parser
stop when parsing no longer consumes input. Add public QueryParser proptests
covering both regressions.
Findings and original fixes by Oleksii Syniakov in osyniakov/tantivy#4.
Co-authored-by: Oleksii Syniakov <1282756+osyniakov@users.noreply.github.com>