Allow lens derive to add consts for custom lenses

Nobody has claimed this yet.

It would be nice if, when writing a custom lens, you could annotate the type so
that the derive macro would include the custom lens along with the derived
lenses. This would look something like (forgive my contrived example):

```rust
#[derive(Lens)]
#[lens(custom(speed_kmph = "lenses::Kmph"))]
struct Arrow {
// a unit vector
heading: Vec2,
// in mph
speed: f64,
}

mod lenses {
struct Kmph;

const KM_PER_MILE: f64 = 1.609_344;

impl Lens for MilesToKm {
fn with R>(&self, data: &Arrow, f: F) -> R {
let kms = data.speed * KM_PER_MILE;
f(&kms)
}

fn with_mut R>(&self, data: &mut Arrow, f: F) -> R {
let mut kms = data.speed * KM_PER_MILE;
let kms_2 = kms;
let result = f(&mut kms);
// avoid doing the conversion if unchanged, it might be lossy?
if !kms.same(&kms_2) {
let miles = kms * KM_PER_MILE.recip();
data.speed = miles;
}
result
}
}
}
```

and then I would be able to access this lenes as `Arrow::speed_kmph`.

The issue names the Rust Lens derive macro, the proposed #[lens(custom(...))] attribute, and the Arrow::speed_kmph entry point; no file or test is provided. Start by locating the derive-macro implementation and relevant lens-generation tests, then establish completion as exposing the custom Kmph lens through Arrow::speed_kmph while preserving its custom behavior.

Written by the indexing model from the issue text.