first pass at lean projections
lean optimize summary and by-of-summary
docstring for _lean
lean optimize on sort
_lean(Symbol) doesn't project if not necessary

Thanks @mwiebe for bringing this up
sort __str__
fix docstring for lean_projection
lean optimize on head
lean: fix head optimization
typo
Merge pull request #740 from mrocklin/lean-projections

Optimization to insert Projections