* Introduce `KeyContainer::compare` and drop the `getKey` method.
* Let users of ZipMapIterator specify a trait class instead of using the
default one.
`ZipMapIterator` allows you to iterate in parallel over two
`std::map`-like containers.
In the ABI analysis, this allows us to be much more efficient. In
practice, if we have two maps with M and N elements, we pass from
performing N*log(N) + M*log(M) queries to the size of the union of the
set of keys of the two maps.
Alessandro Di Federico