Assorted D3D12 suballocation cleanups

[cwfitzgerald]

Connections

Part of my work on #4150.

Description

This PR solves two problems:

• After using the suballocation module in anger, I realized there was quite a lot of excess cruft from an older design. This cleans it up and tries to simplify the API.
• This enables me to create buffers from the command encoder, which is something I need for [d3d12] Support non-default rows_per_image #4150.

Each commit is standalone and the review experience is commit-by-commit.

Testing

CI.

Squash or Rebase?

Rebase vibes.

Checklist

• Run cargo fmt.
• Run taplo format.
• Run cargo clippy --tests. If applicable, add:
  • --target wasm32-unknown-unknown
• Run cargo xtask test to run tests.
• If this contains user-facing changes, add a CHANGELOG.md entry.

[teoxoy]

Nice!