- Remove useless QIRReductor class

translates the QIR tree to an executable QIR tree by detecting where to
execute the query parts. The evaluation then calls the appropriate visitor for
the evaluation of the different parts
- Create package qir.visitor, and reorganization of files

- Remove useless operators
- Minor improvments in GreedyReduceVisitor

- Table name of QIRTable now a QIRNode
- Fix toString for QIRString

- Remove old hack in PostgreSQL driver
- Rename QIRSelect in QIRFilter to avoid confusions with SQL's select

- Minor fixes

- Better toString for QIRLambda
- Throw exception in case of error in execution of query

- Add getDriver for QIROperators

- DBDriver is now generic, rest merged with SQLDriver
- Add QIRException
- Use final everywhere it applies

- Fix QIRFreeVarsVisitor

- Add QIRFreeVarsVisitor to find the free variables in a QIR tree
- Add SQLDriver as abstract driver to factorize code between drivers for SQL
databases
- QIRVariable now contains a FrameSlot from Truffle
- QIRTable now contains a DBDriver to refer to a targeted database
- QIRReductor now returns a QIROperator