Expose derived lenses as associated constants

[Ralith]

Generates lens type definitions inside a private module with an unlikely-to-collide name, then uses those to populate associated constants on the type for extremely ergonomic access. The main drawback here is that the private __Foo_druid_lenses module name pops up in error messages.

[Ralith]

Updated to define the lenses in a public module with some documentation. This makes things a little prettier; it's not perfect but I think it's pretty good.

Example error from using a lens of the wrong type:

error[E0277]: the trait bound `impl druid::Widget<f64>: druid::Widget<bool>` is not satisfied
  --> druid/examples/slider.rs:53:37
   |
53 |     col.add_child(Padding::new(5.0, slider), 1.0);
   |                                     ^^^^^^ the trait `druid::Widget<bool>` is not implemented for `impl druid::Widget<f64>`
   |
   = note: required because of the requirements on the impl of `druid::Widget<DemoState>` for `druid::lens::LensWrap<bool, demo_state_lenses::double, impl druid::Widget<f64>>`
   = note: required by `druid::widget::padding::Padding::<T>::new`

[cmyr]

My only nit might be to add generated or derived to the name of the module; so something like demo_state_generated_lenses or demo_state_derived_lenses?

[Ralith]

We'll be able to get rid of the public module when rust-lang/rust#63065 comes along.

[cmyr]

want to make that name tweak? then I'm happy to merge, this is a solid improvement.

[Ralith]

Done, plus some brief documentation.

[cmyr]

great, thanks, appreciate the attention to detail.