Speed up Manifest#clean by reducing complexity
Speed up Manifest#clean by reducing complexity

Change cleaning old compiled assets from using a O(n^2) to a O(n)
algorithm for finding old files to remove. This matters since there
can be thousands of assets.

recursecenter/sprockets,guilleiguaran/sprockets,kappezoro/sprockets,eileencodes/sprockets,masarakki/sprockets,jcoyne/sprockets,daniel-rikowski/sprockets,maclover7/sprockets,sideci-sample/sideci-sample-sprockets,jeremy/sprockets,basecamp/sprockets,askl56/sprockets,prusswan/sprockets,sferik/sprockets,byroot/sprockets,prathamesh-sonpatki/sprockets,Andreis13/sprockets,bdezonia/sprockets,rails/sprockets,orlando/sprockets,tricknotes/sprockets,JuanitoFatas/sprockets