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/codegen.go

@@ -410,6 +410,9 @@ func (c *codegen) Visit(node ast.Node) ast.Visitor {
 					if !multiRet {
 						c.registerDebugVariable(t.Name, n.Rhs[i])
 					}
+					if t.Name != "_" {
+						c.scope.newLocal(t.Name)
+					}
 					fallthrough
 				default:
 					if i == 0 || !multiRet {

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))
+}