compiler: implement shadowing of global variables

Before introducing slots it was hard to change global variables
preserving changes across multiple function calls.
This commit implements such possibility.
Closes #638.

pkg/compiler/global_test.go

@@ -0,0 +1,21 @@
+package compiler_test
+
+import (
+	"math/big"
+	"testing"
+)
+
+func TestChangeGlobal(t *testing.T) {
+	src := `package foo
+	var a int
+	func Main() int {
+		setLocal()
+		set42()
+		setLocal()
+		return a
+	}
+	func set42() { a = 42 }
+	func setLocal() { a := 10; _ = a }`
+
+	eval(t, src, big.NewInt(42))
+}