Implement operator for unqualified `?s ?p ?o`

[gtfierro]

This should be easy to implement because its just iterating over the underlying LevelDB, but we need to give some thought on how to iterate:

do we list the triples as they were inserted?
do we only want to list triples that are in the building?
do we want to ignore the 'inverse' triples?

[gtfierro]

Done