Reorganize verified double specs.

The single `verifying_double_spec.rb` has gotten unwieldy and hard
to work with because it's so large. It's easier to work with smaller
spec files.

- Extract doubled class definitions into a support file.
- Move the method visibility specs into their own file.
- Move class/object/instance double specs into their own files.
- Move general verifying double construction specs into a separate file.
Make doc strings a bit more accurate.
Rewrite verifying message expectation specs.

- Move them under `verified_doubles` sub directory.
- Use more of an integration style rather than a mock-based
  unit style.
- This is easier to work with; the other specs surprised
  me in that I had to explicitly expect the error generator
  not to receive a message in order to write a spec for the
  "allowed" case (since it was `as_null_object` it would not
  fail automatically). This surprised me when trying to add
  some new specs here.
- This also fixes the spec for the `any_args` case. The old spec
  did not fail when I temporarily removed the special `any_args`
  handling, because the expected method for that example accepts
  1 argument, and without the `any_args` special handling, it would
  naturally be treated as a single argument.