derive support for enums with newtype/unit variants

[jordens]

• Only newtype and unit variants supported currently. Non-newtype tuple structs can be used with skip fields. Both record structs and non-newtype tuple structs would require creating proxy structs on the fly to implement KeyLookup and TreeKey on them.
• The .0 newtype hierarchy level is flattened intrinsically.
• Unit variants are supported by being equivalent to skip. They would only become relevant once there is some instrinsic support for switching variants.
• No intrinsic support for variant read. An external getter can be used.
• No intrinsic support for variant write. An external setter can be used. This needs some user code in any case since there is no Default per variant.
• The implementation differs from the one proposed in derive macro for complex enums #166 in that even for depth = 1, there is no intrinsic support for variant switching.
• No support for variant flattening. This is the only thing that would be missing to support Option-style behavior through the derive macros.

[jordens]

@MathiasKoch does this help you? There are some examples in tests/enum.rs.

[MathiasKoch]

This looks amazing!
I never got around to implementing it myself yet, but this will absolutely help!