Bring query-json closer to jq

[np]

• Internally filters can now emit multiple outputs.
• These constructs are now supported:
  • empty
  • expr1 , expr2
  • (expr)
  • [expr]
  • select(expr)
  • not
• filter(e) is now a shortcut for map(select(p))
• Conditional are just expressions (they return a JSON boolean value).
• Extend == and != to all JSON values
• Fix priorities according to jq
• Fix paths: .foo[4] and .[4] are paths [4] is not a path
• Fix the parser to follow better the jq grammar (expr vs term, no conditionals, no paths)
• Extend parsing tests