Support for backward/forward compatibility

[durban]

Currently Compat[A, B] and Model#compatible(a, b) are symmetric (in A/B and a/b respectively). There are cases where it would be useful to have a "partial" compatibility relation. For example, we might only care that we can read serialized data from an older version, but we don't require that that older version be able to read current serialized data. For these cases, it would be good to have something like ForwardCompat[OLD, NEW] (and Model#forwardCompatible). And/or BackCompat[NEW, OLD] could be equivalent. (See also this comment.)