Introduce SharedRef<T> and UniqueRef<T>

[Thealexbarney]

The SharedRef<T> struct allows reference counting objects. UniqueRef<T> ensures that an object always has a single owner and is disposed when it's not used anymore.

These types must never be copied and must always be disposed of properly. The NonCopyableDisposable attribute is used to mark these types.

A non-copyable-disposable (NCD) type has a few rules to ensure it's used properly:

1. An NCD local variable must always be a using variable.
2. An NCD should not be copied in any way, including copying structs that contain an NCD.
3. An NCD field must not be assigned outside of a constructor. Methods like .Set() should be used instead.
4. An NCD type must always be passed by reference.
5. An NCD type may be returned from a function if the return statement directly returns a new NCD instance or if it's invoking another function that returns an NCD.
6. Structs and classes containing an NCD should be disposed of properly, and should dispose of their NCD fields.
7. A reference to an NCD must not be copied to a value.
8. References to an NCD may be returned and assigned to variables freely.