rewrite call and foreach

before:
% ./run_integration_test.rb android
tar -xzf ../android.tgz
Running make for android... 6.05 secs
Running kati for android... 64.08 secs
android: OK

PASS!

after:
% ./run_integration_test.rb android
tar -xzf ../android.tgz
Running make for android... 5.76 secs
Running kati for android... 41.54 secs
android: OK

PASS!

1 files changed, 1 insertions, 0 deletions

diff --git a/var.go b/var.go
index fef5728..1bb93d2 100644
--- a/var.go
+++ b/var.go
@@ -15,6 +15,7 @@ type Var interface {
 }
 
 type SimpleVar struct {
+	// TODO(ukai): string -> Value (literal or so?)
 	value  string
 	origin string
 }