compiler: create eval vm lazily

compiler.go

@@ -129,7 +129,6 @@ func newCompiler() *compiler {
 
 	c.newScope()
 	c.scope.dynamic = true
-	c.evalVM = New().vm
 	return c
 }
 

compiler_expr.go

@@ -995,6 +995,9 @@ func (c *compiler) evalConst(expr compiledExpr) (Value, *Exception) {
 	if expr, ok := expr.(*compiledLiteral); ok {
 		return expr.val, nil
 	}
+	if c.evalVM == nil {
+		c.evalVM = New().vm
+	}
 	var savedPrg *Program
 	createdPrg := false
 	if c.evalVM.prg == nil {