Context manager for various traits

[vreuter]

This is an alternate provision of different behaviors that will be preferable relative to a different subtype for some usage patterns. Rather than create a more specific instance with the trait mixed in, a developer could use a particular behavioral context, upon exit from which the map's behavior set would revert to what it was prior to entering the context.

[vreuter]

An ultimately better (often, anyway) solution to #12