Typing gets its context from the model, not the AST

This replace some usage of AST nodes with model entities in the signature of some services in modelbuilder and typing.

This simplifies the clients that can now think less about the AST.
But the main objective is to permit, in some reasonable future, to feed arbitrary piece of code to the phase system.

Eg. of concrete future use: safe and clean analysis of pieces of code in documentation.