Refactor the code to have a unique place for signature definition.

Re-architecture the code to have all signatures in the sigs.ml module.
Any signature which occurs at least twice in the code (e.g. in a.ml
and corresponding a.mli) is put in a module A in Sigs. The signature
can the be included in a.ml and its 'module type of' can be included
in a.mli.

private signatures can stay in the .ml files where they appear.