fix(cdk): Fn.cidr should return a list and not a string (#2678)

CloudFormation's Fn::CIDR returns a list of CIDR blocks and not a string.
So we need to use a listified token instead of a string.

Fixes #2671

packages/@aws-cdk/cdk/lib/fn.ts

@@ -112,8 +112,8 @@ export class Fn {
    * @param sizeMask The digit covered in the subnet.
    * @returns a token represented as a string
    */
-  public static cidr(ipBlock: string, count: number, sizeMask?: string): string {
-    return new FnCidr(ipBlock, count, sizeMask).toString();
+  public static cidr(ipBlock: string, count: number, sizeMask?: string): string[] {
+    return new FnCidr(ipBlock, count, sizeMask).toList();
   }
 
   /**