[naga] Fix improper feature gates

[bushrat011899]

Connections

• Contributes to Support no_std where possible (specifically for wasm32v1-none) #6826

Description
The arbitrary feature of half was always enabled, even when naga's own arbitrary feature was not enabled. Likewise, num-traits had its default features enabled even though they weren't being used. These issues will add noise to no_std efforts in naga in the future.

To address this, I have disabled default features in half and num-traits, and ensured half/arbitrary is enabled when naga/arbitrary is.

Testing

CI

Squash or Rebase?

Squash

Checklist

• Run cargo fmt. (Not Applicable)
• 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. (Not Applicable)