Extract InCompartment API helper from Servo

[jdm]

https://github.com/servo/servo/blob/da559d47b93854e4c7d4acf0a0d2029a9e643794/components/script/compartments.rs

We should bring the AlreadyInCompartment and InCompartment types into this crate (renaming them to AlreadyInRealm and InRealm in the process), and make the assert method take a *mut JSObject object instead. This will allow us to add a InCompartment argument to ToJSValConvertible, avoiding a source of crashes.

[jdm]

We could also add it to the typed array construction API in

rust-mozjs/src/typedarray.rs

Line 268 in 9b0d063

unsafe fn create_new(cx: *mut JSContext, length: u32) -> *mut JSObject;

.