`yield*` macro

[Mercerenies]

A yield* macro that yields from whatever function you call until the function terminates. We could compile it something like (rough example below)

(yield* (foo))

to

(let ((result (foo)))
  (while (and (instance? result GDScriptFunctionState) (result:is_valid))
    (yield)
    (setq result (result:resume)))
  result)

which would (barring weirdness with and requiring the while intermediate form right now) compile to basically

var result = foo()
while result is GDScriptFunctionState and result.is_valid():
    yield()
    result = result.resume()
return result

[Mercerenies]

Closed as of 9867fe7. There's some some room to optimize the resulting GDScript, as it is definitely more complicated than the proposed GDScript above, but the GDLisp expansion is identical.