Bumps [smallvec](https://github.com/servo/rust-smallvec) from 1.5.0 to 1.5.1.
- [Release notes](https://github.com/servo/rust-smallvec/releases)
- [Commits](https://github.com/servo/rust-smallvec/compare/v1.5.0...v1.5.1

)

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