Representing enums in the ui

Update: See https://github.com/Finnerale/druid-enums for an implementation of the derive.

I've opened this Issue to continue the discussion about this from #751 and zulip

Currently druid does not offer a good solution to representing enums in the ui.
While structs can be split up into their different fields using Lens, no such feature exists for enums.
Considering how prevalent enums are in Rust, I would argue that druid should provide a good solution for this.

The ideas so far are

1. Extending Lens to work with enums
2. Introducing a new derive specifically for enums
3. Using an immediate mode widget (#751)

I've implemented (examples of) those three here,
also @cmyr expressed similar ideas on #751

My preferred solution would be number 2, introducing a new Match derive similar to Lens which adds a ::matcher() -> MyTypeMatcher function to the deriving type (MyTypeMatcher would be generated).
For example if Option would derive Match:

Option::matcher()
    .some(|| make_widget_some())

Having the matcher as a function on the type should be more intuitive than having to know that there is now also a separate struct called OptionMatcher, especially when exporting (pub use) it.
This matcher would provide one method per variant, as well as a default method to mimic _ =>.

Of course this does not event get close to the power match provides but it should be good enough for most cases. Would be nice if we could collect some ideas here and come up with something truly satisfying.