Optimize Next implementation

Once the program detects that we have stepped into another function,
we simply calculate the return address and then set a breakpoint and
continue to that location, avoiding numerous syscalls.

_fixtures/testnextprog.go

@@ -37,7 +37,7 @@ func testnext() {
 func main() {
 	runtime.LockOSThread()
 	for {
-		sleepytime()
+		helloworld()
 		testnext()
 	}
 }