Properly move key when moving Track

The following discussion from !1084 (closed) should be addressed:

• @graven started a discussion: (+4 comments)

  Why change from ScalarTransformer to Transformer? Ideally, whatever the reason, ScalarTransformer should be 'fixed' / 'augmented' / ... to support this use-case...

Summary of the discussion:

• Leave problem in Track v1, fixing it might introduce other headaches
• Drop dependency on KeyedObject in (upcoming) Track v2.
• Specify move constructors noexcept so that they can be used when resizing vectors.