In particular, its interface might still change (in interaction with interested
 colleagues). So let's not give it too much visibility yet. Instead, I'll turn
 it as an opam packages for now.

 Most of them are backports of improvements already there in
 FSetPositive when compared with the original FMapPositive file.

 NB: this is work-in-progress, there is currently only one
 provided implementation (MMapWeakList).

 In the same spirit as MSets w.r.t FSets, the main difference between
 MMaps and former FMaps is the use of a new version of OrderedType
 (see Orders.v instead of obsolete OrderedType.v).

 We also try to benefit more from recent notions such as Proper.

 For most function specifications, the style has changed : we now use
 equations over "find" instead of "MapsTo" predicates, whenever possible
 (cf. Maps in Compcert for a source of inspiration). Former specs are
 now derived in FMapFacts, so this is mostly a matter of taste.

 Two changes inspired by the current Maps of OCaml:
 - "elements" is now "bindings"
 - "map2" is now "merge" (and its function argument also receives a key).

 We now use a maximal implicit argument for "empty".