Generic maskedstorage

[Rua]

Fixes #664

API changes

This changes three things:

• UnprotectedStorage has its item type as an associated type rather than as a generic type parameter.
• The type parameter of MaskedStorage is now the inner storage, rather than the item type.
  This allows any item to be stored in a MaskedStorage, rather than only Components.
• MaskedStorage now has contains, get and get_mut methods.

[Rua]

There's two errors remaining that I can't figure out. They seem to have to do with the definitions of ReadStorage and WriteStorage, which are type aliases. If I substitute them for their definitions at the site of the error, the error goes away.