SEARCH: improve stage reordering

[jacoscaz]

The current method is inefficient (too many wasted cycles and allocations). It's a start but it will need to be refactored into something better once we get a better grasp of whether we want to keep a flat array-like structure for search stages or whether we'll be forced to adopt a recursive structure like sparqlalgebrajs's output.

[jacoscaz]

Closing as a consequence of #104