Implement Piet interpreter

## Implemented Piet
* [x] [Original Piet Interpreter](https://github.com/helvm/npiet) - `Erik Schoenfelder has written npiet, a very nice Piet interpreter with the ability to print program traces and a supporting Piet editor! It's feature-packed and pretty cool.`mall
* [x] https://github.com/helvm/piet-willem-o - An interpreter for the programming language Piet (http://www.dangermouse.net/esoteric/piet.html). Currently only runs the program "hi.png".
* [ ] https://github.com/helvm/pietc - pietc is a Piet compiler. This compiles Piet programs to native executable programs.
* [ ] https://github.com/helvm/piet-interpreters - it doesn't compile

## Features
* [ ] use ppm P3 and P6
* [ ] use xpm
* [ ] use llvm-gen
* [ ] rewrite show for flatten list and nested list. Main tests for evaluator. Emit flatten list, nested list and raw code
* [ ] rewrite original Piet to use one grid with all data in structure
* [ ] rewrite pietc to generate grid
* [ ] dup, load ->
* [ ] cons a, cons a, load

## Require
* https://github.com/helvm/helma/issues/7
* https://github.com/helvm/helma/issues/37
* https://github.com/helvm/helma/issues/39