Positive decomposition

[WoutPiessens]

Building on #924 and #929, this is a first implementation of the separate positive decomposition for global constraints.
For now:

• decompose_positive() is called if the global constraint c is top-level or positively reified (b => c). In case the global constraint is nested in any other way, it is decomposed as it was before.
• Only the MDD global constraint supports a separate positive decomposition

[tias]

reviewed in #981