Return references to objects, where appropriate

[hannobraun]

Update the kernel APIs that return objects, to return references to them instead, where appropriate. This has the following advantages:

• It makes the potential cost of copying/cloning the object explicit.
• It avoids the cost of copying/cloning the object, where that's not needed.
• It helps in some use cases, for example when working with trait objects, where your only alternative to using a reference is putting them in a Box (or similar).