compiler: support returning values from async functions

This is implemented as follows:

  * The parent coroutine allocates space for the return value in its
    frame and stores a pointer to this frame in the parent coroutine
    handle.
  * The child coroutine obtains the alloca from its parent using the
    parent coroutine handle. It then stores the result value there.
  * The parent value reads the data from the alloca on resumption.

testdata/coroutines.txt

@@ -7,6 +7,7 @@ wait:
   wait start
   wait end
 end waiting
+value produced after some time: 42
 non-blocking goroutine
 done with non-blocking goroutine
 async interface method call