This patch solves a problem on the order of the generation of new
declarations by intro_projections_counterexmp. The problem came from
the combination of Trans.on_tagged_ls and Trans.decl. The former was used
to know each projection function in advance and the latter would
introduce new declarations directly at the definition of the decl being
analysed sometimes using projections functions before they are defined.

This patch makes intro_projections_counterexmp.ml adds declarations only
at the end of the task. So we are sure that any meta is really defined
before.

* src/transform/intro_projections_counterexmp.ml
 (introduce_constant): Minor clarification.
 (projections_for_term): Replacing append with proper symbol. Also adding
  already computed list of declaration to the output.
 (introduce_projs): Removing already defined declarations from the set of
  declaration we will add.
 (build_projections_map): The order of elements is irrelevant so
  it is faster to use cons instead of append.
 (meta_transform2): Add declarations created by f at the end of the task.
 (encapsulate): Compose meta_transform2 and introduce_projs.
 (commented meta_transform2): Request for a fold able to do what is
  described.