Refactor crate to be more composable and simple

[Kimundi]

Right now the crate contains a bunch of types and typedefs for different mutabilty and usecase scenarios.

It would be nice if the API could be simplified by offloading some of the duplication and different behaviors to type paramers instead.

Possible refactors:

• Anything using the Erased trait should be made generic over the actual trait, to support varitions like Erased+Send or Erased+Sync
  • This especially includes the *Ref type defs
• OwningRef and OwningRefMut are potentially mergeable by adding a type parameter for the mutabilty: OwningRef<*const T> and OwningRef<*mut T>. Or with specially defined marker types: OwningRef<Shared> and OwningRef<Mutable>.
• OwningRef might possibly be implemented in terms of an OwningHandle.

[Storyyeller]

I don't think it's useful to try to merge OwningRef and OwningRefMut, since Rust doesn't provide any good way to abstract over mutability. Even if you try, you'll run into the issue that methods have to change receiver type (&self vs &mut self)