Ruby name conventions.
Avoid rebuilding the parser all the time.
Fix Operator#==
Refactor optimization architecture; start optimization of projections.
Avoid checking this twice.
Added TupleComputation#empty?
Added TupleComputation#project
Optimize extensions.
Optimize project(union)
Optimize projections of join/matching/not_matching
Optimize projections of sorts.
Make Project optimizer complete through on_todo.
Optimize projection of rename.
Add a test on unoptimizable restrict(rename)
Remove TypeSafe and InferHeading operators.
Add type checking of relational expressions.