Expand Lens / Lens derive functionality

#118 landed an absolute barebones version of Lens derive. There's also been some discussion in Zulip about a [Lens2](https://git.sr.ht/~raph/interp-toy/tree/master/src/lens2.rs) and whatever that might entail.

Also, as @cmyr mentioned in #118:
> As future work we're also going to want to support (at the very least) deriving lenses into collections; in this case the concrete type of the Lens will need a field for the index of the collection we want.
> This also makes lenses fallible (you might have an invalid index), which is something we'll want to to think about; we want to at least handle this without crashing.

One more also: I ended up dropping the `lens_wrap!` macro because it's unclear what the ideal usage should be.

Here's an alternative @cmyr mentioned in the PR:
```rust
impl Point {
fn lens_width(&self, inner: I) -> impl LensWrap, I> {
druid::LensWrap::new(inner, lenses::point::width)
}
}

let wrapped_widget = Point::lens_width(my_widget);
```

*What would help:* a concrete an example of a Lens on a collection, and any further ideas on LensWrap.

Read the existing Lens derive work from #118 and inspect src/lens2.rs for the proposed Lens2 direction. Define a concrete collection-lens example and resolve the intended LensWrap API; done requires an agreed design for collection indices, fallible access, and wrapping.

Written by the indexing model from the issue text.