Implement the lint_too_many_instance_attributes Warning

[GreasySlug]

Fixes #500

Changes proposed in this PR:

• add lint command (cargo rd -- --lint)
  If it is not intentionally provided as a command, I'll delete it.
• warning if instance attribute is 36 or higher

---

Is it possible to take the location of an instance attribute?

If possible, I would like to

• idea

36 |     foo_36 = Int;
   :     ------------
   :     `- take advantage of subclasses and data classes

AttributeWarning: too many instance attributes

@mtshiba