Implemented variable scopes

src/build/core/CompileAssign.go

@@ -8,9 +8,9 @@ import (
 // CompileAssign compiles an assign statement.
 func (f *Function) CompileAssign(node *ast.Assign) error {
 	name := node.Name.Text()
-	variable, exists := f.variables[name]
+	variable := f.Variable(name)
 
-	if !exists {
+	if variable == nil {
 		return errors.New(&errors.UnknownIdentifier{Name: name}, f.File, node.Name.Position)
 	}