Now that we only have main thread contexts, we can rethink how JSContext and JSRuntime are related.

Years ago in bug 1281529 I made JSContext inherit from JSRuntime. This was nice because it gets rid of a dereference to access data in the runtime. The runtime could still be used for things that are accessed on helper threads too but main-thread only data would be stored in the context. Unfortunately this change was reverted as part of the cooperative scheduling work that has since been removed, but we're now in a position to redo this.