Make parser unaware of declaration ordering

[ogrergo]

At the moment, the order of declaration (@node, @word, @ParaNode etc...) matter : if a node depends to a foward declared node, the parser will return an error.

The goal is make the parser support disordered declaration.

This will implies adding an extra step of contextual check on the AST. This new pass goal will be to build the dependency tree between PathTree. That dependency tree will be then used by the check_* contextual pass to perform the checking on Identifier AST.