Implement GenericReadStorage for mutable reference

[richard-uk1]

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

[richard-uk1]

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