Reduce string allocations related to labels and logging.

[kpreid]

Description

• Resource::label() is (occasionally, for destroyed resources specifically) called for logging, but always allocated a String, which would not necessarily be skipped by the logging macro (if the level filter is met, a format_args!() is constructed even if it is skipped). Avoid that by making it return &str instead.
  • This has one behavior change: wgpu_core::command::CommandBuffer will return the "<CommandBuffer>" placeholder string instead of "". This avoids taking a lock in addition to the allocation, but it also means Registry::label_for_resource() won't produce a placeholder with ID. But if that behavior is desirable, surely it should be done more broadly (have Resource::label return empty strings consistently), instead of only for CommandBuffer?
• Also avoid allocating a throwaway String when creating an unlabeled CommandBuffer.

Testing

Ran cargo xtask test. All failures on my machine are unrelated to label strings as far as I know.

Checklist

• [X] Run cargo fmt.
• [X] Run cargo clippy. If applicable, add:
  • [ ] --target wasm32-unknown-unknown
  • [ ] --target wasm32-unknown-emscripten
• [X] Run cargo xtask test to run tests.
• [X] ~~Add change to CHANGELOG.md. See simple instructions inside file.~~ Not worth mentioning.