Realm C Bindings

[lightsighter]

It's looking increasingly likely that at some point in the near future we might need Realm C bindings. Our approach to Realm C bindings should be somewhat similar to what we do in Legion, which is we don't try to do the whole API all at once, but we create a general framework for adding bindings and then people can add in new bindings on demand as they need them. To start this process, we should seed the bindings with some of the more common operations that a Realm user might want to do. I feel like these should include the following types and APIs.

• Runtime::start/init
• Runtime::shutdown
• Runtime::wait_for_shutdown
• Event::merge_events
• UserEvent::create_user_event
• UserEvent::trigger_event
• Processor::register_task
• Processor::spawn
• PhysicalInstance::create_instance
• IndexSpace (constructor, will probably need rect/point types)
• IndexSpace::fill
• IndexSpace::copy
• Machine Processor Query
• Machine Memory Query

I think this should be a core minimal subset of the Realm API to allow people to start writing interesting programs and new bindings can be added on demand as needed. Assigning @apryakhin for triaging (feel free to delegate).