Encoding of the occurences using Huffman

Encoding the list of occurences in one single list of bytes using the Huffman
codes generated. Later, this list of bytes (which contains either 1 or 0) will
have to be output properly to the compressed file. i.e. each byte represent
one bit for now.

- Test in consequences in AstCompressorTes.scala
- Addition of a dummy (larger) program in sandbox to see it working on real
programs.