Add attributes to objects in runtime

[kondziu]

When executing a query, sometimes people want to calculate and add an attribute to the object. We should perhaps add a facility to do so. Objects and their existing attributes should ultimately remain read-only, so these would be volatile.

Perhaps we could add a map to each object that could be added to within the DSL.

Ideas:

database.projects().attach_attribute(Attribute("HasTorvalds"),  Contains(project::Authors, User::new("torvalds", "...@...")));
database.projects().attach_attribute(Attribute("HasTorvalds"), |project| project.users().iter().any(|user| /* .. */.));