Create InstantiatedModule type

[codefromthecrypt]

There's a lifecycle requirement that is tricky to prove here:

The start function is intended for initializing the state of a module. The module and its exports are not accessible before this initialization has completed.

https://www.w3.org/TR/wasm-core-1/#start-function%E2%91%A0

Right now, Store includes the instantiated module state internally.

	err = store.Instantiate(mod, "test")
        store.CallFunction("test", "fac-iter", in)

Due to this, to satisfy the above things, runtime checks like this have to happen internally.

	m, ok := s.ModuleInstances[moduleName]
	if !ok {
		return nil, nil, fmt.Errorf("module '%s' not instantiated", moduleName)
	}

This are subject to racing which makes them hard to test also. Finally, it burdens the user a little as they have a way to invoke something when it can be in the wrong state. Ex.

	// forgot to call store.Instantiate, or some other goroutine isn't done that.
        store.CallFunction("test", "fac-iter", in) // bombs

I think a cleaner design would be to make an InstantiatedModule type and move features that require start being invoked first, to be on that type. That features are only on the correct type, people wouldn't be tempted to do something at the wrong time or when something isn't available

Ex.

	store.CallFunction("test", "fac-iter", in) // compile error!
	imod, err = store.Instantiate(mod, "test")
        imod.CallFunction("fac-iter", in) // compile error!
        factIter, ok = imod.GetFunction("fac-iter") // if ok, definitely that function exists
        factIter.invoke(ctx, in) // naming this "call" is also fine. whichever you can use it as often as you like

[codefromthecrypt]

If we do this I also think we should think hard about whether adding and replacing functions on an instantiated module is something to support or not. Safety-wise it feels we shouldn't keep doing this, rather allow someone to change the imports/exports etc only up to .Instantiate(), then after that they are fixed.

[codefromthecrypt]

from @mathetake

let's add store.GetModuleInstance as well or return *ModuleInstance on store.Instantiate to line with moduleInstance.GetExport API 😄

[codefromthecrypt]

this is done via api.Module