Introduce new operators for streaming filters len[e,g,ge,l,le]

These new operators can match depending the length of a field.
This way we can describe condition like: "filter for replies" because an annotation is reply when len(references) > 0
New operators were missing from the operators map
Make operator symbols a bit more consistent
Regression fix: Operator mismatch was possible due to the bad order
Merge pull request #1055 from hypothesis/new-streamer-operators

New streamer operators