Detect unnecessary `stub_const` calls

[dgollahon]

Check for unecessary stub_const (see rubocop/rubocop-rspec#423 (comment))

See #16 as well

[dgollahon]

Might be a job for a different tool but I also wonder if it should detect:

• Creating intermediary classes (instead of triggering an autoload)
• Stubbing a constant that doesn't actually exist