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.
- Implementation of a tree recomposer, which will be used for the
decompression. For now in Plugin.scala.
- Tentative implementation of a classic Lzw compression for names (will have
to be updated later, depending of how symbols are handled).