engine: add simplistic ExternalReferences type to manage external references

[deadprogram]

In similar spirit to #6 this PR introduces ExternalReferences type to manage external references managed by host, but used from guest modules.

This would be the host side:

thing := testingType{val1: "hello", val2: "world"}
ref := eng.Interpreter.References.Add(unsafe.Pointer(&thing))

instance.Call("here_is_thing", ref)

The guest side:

type externref uintptr
var thing externref

//go:wasmimport host foo
func foo(what externref)

//go:wasmimport host bar
func bar(what externref)

//go:export here_is_thing
func here(something externref) {
    thing = something
    foo(thing)
}

//go:export sometime_later
func sometime() {
    bar(thing)
}

[orsinium]

The most canonical way to do that is through a table of externref type. IDK if it's supported by wasman from the host side and by tinygo from the guest side but it's certainly the most close to the wasm spec solution.

[deadprogram]

@orsinium I will look into that possibility more fully. However, I think that there is not yet anything exposing that to the guest side in TinyGo.

[deadprogram]

After reading a bit more, I think we should not base providing this capability on table implementation of externref.

Most actual use cases in the wild I could find receive an externref back from some host function, and then just use as a param when calling other host functions.

Separately, I added a more random way to generate a new reference ID.

[orsinium]

I'm curious to see where and how Remove will be called. It's easy to leak the references.

[deadprogram]

I'm curious to see where and how Remove will be called. It's easy to leak the references.

Likewise. Let's find out...