them.

Today, we sort all entities added to all phases, even the phases that
don't strictly need sorting, such as the opaque and shadow phases. This
results in a performance loss because our `PhaseItem`s are rather large
in memory, so sorting is slow. Additionally, determining the boundaries
of batches is an O(n) process.

This commit makes Bevy instead applicable place phase items into *bins*
keyed by *bin keys*, which have the invariant that everything in the
same bin is potentially batchable. This makes determining batch
boundaries O(1), because everything in the same bin can be batched.
Instead of sorting each entity, we now sort only the bin keys. This
drops the sorting time to near-zero on workloads with few bins like
`many_cubes --no-frustum-culling`. Memory usage is improved too, with
batch boundaries and dynamic indices now implicit instead of explicit.
The improved memory usage results in a significant win even on
unbatchable workloads like `many_cubes --no-frustum-culling
--vary-material-data-per-instance`, presumably due to cache effects.

Not all phases can be binned; some, such as transparent and transmissive
phases, must still be sorted. To handle this, this commit splits
`PhaseItem` into `BinnedPhaseItem` and `SortedPhaseItem`. Most of the
logic that today deals with `PhaseItem`s has been moved to
`SortedPhaseItem`. `BinnedPhaseItem` has the new logic.

FPS results are as follows:

| Benchmark                | `binning` | `main`  | Speedup |
| ------------------------ | --------- | ------- | ------- |
| `many_cubes -nfc -vmdpi` | 4.608     | 3.373   | 36.61%  |
| `many_cubes -nfc`        | 42.161    | 35.437  | 18.97%  |
| `many_foxes`             | 308.943   | 303.321 | 1.85%   |

(`-nfc` is short for `--no-frustum-culling`; `-vmdpi` is short for
`--vary-material-data-per-instance`.)

This commit fixes two bugs:

1. Shadow bin keys didn't contain the mesh IDs, causing us to
   incorrectly batch things together that shouldn't have been.

2. Splitting batches for dynamic uniform IDs caused `batchable_keys` and
   `batches` to get out of sync, messing up the rendering in
   `many_cubes` on WebGL 2. This fixes the issue by explicitly storing
   both instance ranges and dynamic uniforms for both batchable and
   unbatchable items.