Simplify the return value of map_difference() to return set of keys.
This makes it possible to use the function with value types such as
foreign_ptr<> that are non-copyable.
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
If map value type is lw_shared_ptr<T>, for example, we need to be able
to pass our own value comparison function to the difference() function.
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
This implements Maps.difference() helper function in C++. We need it to
translate various Origin call-sites that use it. The implementation is
written from scratch with Guava code used as reference to preserve
semantics.
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>
