nest-struct filed support

Big Bang:

• support string constraint
  • tags: min, max, default, req, regex, in
• pointer-to-number
  • such as, *int8, *uint64...
  • tags: min, max, default, req
• nest struct field, or pointer to struct object
  • tags: req

fix small bug

• support number struct filed, tags:
  • min
  • max
  • default