[Discussion] Avoid leaking internal temporary slots from registry entries

[caridy]

@guybedford mentioned something concerning a while ago, the fact that existing API will allow a user to inquire the loader about a particular module to get access to some internal slots from the registry entry, which included a wrapping promise for its various states, which implies that we will have to hold forever any reference to the source text, and all the internal stuff we need to instantiate a module.

In PR #65 we have started the preliminar work to abstract that out by introducing statePromise alongside state value, to only expose the promise associated to the current state. In theory, we should be able to collect promises from previous states along the way. We will need more iteration on this.

[caridy]

initially addressed by #65

[caridy]

this is solved now, and #97 will make it every simpler.