Rework the project architecture

Adopt the code architecture of !1 (merged) in the prototype to see if it makes sense.

This MR does a few renamings and moves code to follow the architecture outlined in !1 (merged).

In particular, it introduces a distinction between language description (meant to be concise because it will be written by humans) and the data model of the Michelson language (meant to be detailed because all backends start from this). As a first step, these two views on the Michelson language are almost identical (and the function to go from the description to the data model is almost the identity); the only difference is in the way the dependency on type arguments for the comparability attribute is declared (a list of type argument in the description versus one or two type arguments in the data model).