Add support for custom types in controls.

[hayleigh-dot-dev]

Currently our controls are built around a Value type that wraps primitives like strings and booleans. It'd be nice if, for example, the user could have a select control that selects from one of their custom type variants instead.

Ideally the api is something like

use value, set_value <- fable.custom_select("wibble", wibble.to_string, wibble.from_string, [
  #("Foo", Foo),
  #("My bar option", Bar),
  ...
])

...