Feature #2597 -- eliminate OpenStruct for performance...

A bit of profiling shows that most of the time spent in clustering is spent
trolling around through OpenStruct. Replacing this with a hash or, where
sane, an array for the stack frame in our non-recursive implementations makes
performance significantly faster. (3 seconds to .65 seconds faster.)

I guess those developer niceties do have some cost after all. Better to take
the hit on readability and prefer performance here.