Support "contexts" by inheriting the instance class

[ColonelThirtyTwo]

The "Context" object is meant to provide settings to the instance, such as the ops limit, sandboxing environment, etc. (for example, the wire processor's context provides the default stuff, but the screen's context lowers the ops limit and gives access to the render library)

However, I think it would be more flexible if we instead create subclasses of instances, and providing overrideable methods and attributes (ex. instance.OpsLimit, instance.Environment, etc)

This would help with #116