Bump smallvec from 1.6.1 to 1.7.0 in /src/rust (#6329)

Bumps [smallvec](https://github.com/servo/rust-smallvec) from 1.6.1 to 1.7.0.
- [Release notes](https://github.com/servo/rust-smallvec/releases)
- [Commits](servo/rust-smallvec@v1.6.1...v1.7.0)

---
updated-dependencies:
- dependency-name: smallvec
  dependency-type: indirect
  update-type: version-update:semver-minor
...

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