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).