Small code size improvement

[goodartistscopy]

Objective

• Improve code size slightly

Solution

• The index-less version of ray_mesh_intersection<T>() does not care about the indices type, so any TryInto<usize> will do.
• By avoiding to monomorphize another version of ray_mesh_intersection<T> (with T=usize) when any of the existing one will do, we get a small improvement in code size (and presumably compilation time as well).

Testing

• Built the mesh_ray_cast example, which works as expected

---

The gain is indeed small (0.8kB in dev, 3kB in release and noise (4B) in size-optimized builds)

Showcase

cargo bloat is used to visualize the differences. Profile dev confirms that one less instance of the function is generated:

> # BEFORE (dev)
> cargo bloat --filter "ray_mesh_intersection$" --example mesh_ray_cast
File .text   Size        Crate Name
0.0%  0.0%   856B bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_mesh_intersection
0.0%  0.0%   856B bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_mesh_intersection
0.0%  0.0%   856B bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_mesh_intersection
0.0%  0.0% 2.5KiB              filtered data size, the file size is 263.3MiB

> # AFTER (dev)
> cargo bloat --filter "ray_mesh_intersection$" --example mesh_ray_cast
File .text Size        Crate Name
0.0%  0.0%   856B bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_mesh_intersection
0.0%  0.0%   856B bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_mesh_intersection
0.0%  0.0% 1.7KiB              filtered data size, the file size is 263.3MiB

In release the function is inlined into ray_intersection_over_mesh() which gains 3kB:

> # BEFORE (release)
> cargo bloat --release --filter "ray_intersection_over_mesh$" --example mesh_ray_cast
File .text   Size        Crate Name
0.0%  0.0% 5.3KiB bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_intersection_over_mesh
0.0%  0.0% 5.3KiB              filtered data size, the file size is 100.7MiB

> # AFTER (release)
> cargo bloat --release --filter "ray_intersection_over_mesh$" --example mesh_ray_cast
File .text   Size        Crate Name
0.0%  0.0% 2.3KiB bevy_picking bevy_picking::mesh_picking::ray_cast::intersections::ray_intersection_over_mesh
0.0%  0.0% 2.3KiB              filtered data size, the file size is 100.7MiB

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[mockersf]

sounds good, we already have u16 and u32 versions, no need for a u64 version of it