gong AST : add marshalling / unmarshalling of the stage as a package

[thomaspeugeot]

Situation:

In modelling activities with gongdoc, the abstract part of the model is the
go code of the model and the concrete part is a function whose purposes is to instantiates shapes.
The shapes refers to abstract objects via the doclink syntax that points to go identifiers.

If we want to extend modelling to other meta model, one either continue to persist the abstract part
as go code (which is possible according to the SySML spec 2.5.1 "semantic conformance" §2 Conformance and §6.3)

Alternative:

suppose we escape the semantic conformance requirement in the model, then one have to produce something that can
be referenced from the concrete syntax. In this case, doclink is not possible anymore to point to go identifiers
since they are not go identifiers anymore (no go code).

A solution would be to persist the stage as a package.