* Reverted [pos.mli] to the previous signature, except [Pos.min]
that now takes a labelled parameter [file].
* Reverted [ParserAPI.ml] to the previous signature.
* Reexported [shor_error] in [ParserUnit].
* Changed the modules [LexToken] so that they export one function
to make attributes, the first, additional paramater being "[@"
or "[@@" (and invalid in PascaLIGO).
* Added support in all [ParserLog] for attributes (pretty-printings).
* Added AST nodes [Attr] and [AttrDecl] to PascaLIGO. The simplifier
takes care of hooking them with their respective declarations
or discarding them. (In the future, we should issue a warning for detached
attributes.)
I removed the last top-level effect (the execution of cpp).
The idea is that ParserUnit.ml and each ParserMain.ml get closer
to pascaligo.ml, cameligo.ml and reasonligo.ml, respectively.
