Derive forking for any enum

[mxgrey]

Currently there is built-in support for forking Result and Option types, but users may want to fork custom enum types as well. We could provide a Fork procedural macro (or maybe ForkEnum) that would allow any enum to be forked.

The same macro could also provide an implementation for Unzippable for the enums that it's applied to, where the branches of the unzip are conditionally run based on which variant the enum contained. In this case we might consider renaming the Unzippable trait to Forkable.