Support alternative variable overrides

Craton's variable override model is based on ChainMap (https://docs.python.org/3/library/collections.html#collections.ChainMap), which follows an override model for lookups that is identical to Python's own scoping model.

However, this is not the only choice. Instead the override can be a merge: eg for a list/set, the result is effectively a union of parent and child lists/sets. For two dicts, parent and child, this could be the same as doing parent.update(child).

There are some nuances: what if the collections are not the same type? For the list case, do the parent's items come first? Are duplicates removed?

One possibility is to make this configurable for a given namespace like ansible/, and follow any desired conventions for that namespace.

Any such work should probably subclass ChainMap, but it should otherwise ducktype like a ChainMap.