Separate the TreeLDR language compiler from the RDF quads compiler.

[timothee-haudebourg]

Over time TreeLDR has become a simple concrete syntax for RDF, just like N-Quads or Turtle. However contrary to the last two, the TreeLDR (language) compiler directly builds a TreeLDR model, while the others are first compiled into RDF quads before being submitted to the RDF compiler. This creates two main pathways inside the compiler, with a lot of redundancy.

TreeLDR (the language) should be treated as any concrete syntax for RDF. No more no less.