Parser to generate dictionary for Tree Compression 

- Partial implementation of a compressor for ASTs. Implementation of a parser which generates a dictionary for the compression, which will then be simplified and encoded.
- Simplification of the structure of trees used for the compression (new "Node" case class).
- Implementation of a tree recomposer, which will be used for the decompression. For now in Plugin.scala. Will require some modifications.
- Tentative implementation of a classic Lzw compression for names (will have to be updated later, depending of how symbols are handled).