Add support for extended relational algebra

There are [many other](http://infolab.stanford.edu/~ullman/fcdb/slides/slides5.pdf) types of operations that can be expressed in relational algebra. This should probably end up behind a toggle since ordering requires lists which changes semantics. An example are those below:

* ~~Sorting~~
* ~~Aggregation~~
* Extended projection (expressions)
* Duplicate elimination