Refactor runner.BenchmarkRunner.plan into a generator
Refactor runner.BenchmarkRunner.plan into a generator

The routine is somewhat more naturally written as a generator.

qwhelan/asv,qwhelan/asv,spacetelescope/asv,pv/asv,spacetelescope/asv,airspeed-velocity/asv,airspeed-velocity/asv,pv/asv,pv/asv,airspeed-velocity/asv,qwhelan/asv,spacetelescope/asv,pv/asv,spacetelescope/asv,qwhelan/asv,airspeed-velocity/asv