Implement GenericReadStorage for mutable reference

[derekdreery]

Without this it's not possible to pass &mut WriteStorage<_> as a GenericReadStorage.

Checklist

• [ ] I've added tests for all code changes and additions (where applicable)
• [ ] I've added a demonstration of the new feature to one or more examples
• [ ] I've updated the book to reflect my changes
• [ ] Usage of new public items is shown in the API docs

API changes

[derekdreery]

I'm going to get the PR building the address the checklist.