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.go

@@ -15,6 +15,9 @@ func main() {
 	wait()
 	println("end waiting")
 
+	value := delayedValue()
+	println("value produced after some time:", value)
+
 	// Run a non-blocking call in a goroutine. This should be turned into a
 	// regular call, so should be equivalent to calling nowait() without 'go'
 	// prefix.
@@ -39,6 +42,11 @@ func wait() {
 	println("  wait end")
 }
 
+func delayedValue() int {
+	time.Sleep(time.Millisecond)
+	return 42
+}
+
 func nowait() {
 	println("non-blocking goroutine")
 }

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