Bump smallvec from 1.4.1 to 1.8.0

[dependabot]

Bumps smallvec from 1.4.1 to 1.8.0.

Release notes

Sourced from smallvec's releases.

v1.8.0

• Add optional support for the arbitrary crate (#275).

v1.7.0

• new_const and from_const constructors for creating a SmallVec in const contexts. Requires Rust 1.51 and the optional const_new feature. (#265)

v1.6.1

• Fix a possible buffer overflow in insert_many (#252, #254).

v1.6.0

• The "union" feature is now compatible with stable Rust 1.49 (#248, #247).
• Fixed warnings when compiling with Rust 1.51 nightly (#242, #246).

v1.5.1

• Improve performance of push (#241).

v1.5.0

• Add the append method (#237).
• Add support for more array sizes between 17 and 31 (#234).
• Don't panic on deserialization errors (#238).

v1.4.2

• insert_many no longer leaks elements if the provided iterator panics (#213).
• The unstable const_generics and specialization features are updated to work with the most recent nightly Rust toolchain (#232).
• Internal code cleanup (#229, #231).

Commits

• 0a4fdff Version 1.8.0
• 6d0dea5 Auto merge of #275 - as-com:arbitrary-support, r=mbrubeck
• 9bcd950 Add support for arbitrary
• 7cbb3b1 Auto merge of #271 - saethlin:drain-aliasing-test, r=jdm
• 0fced9d Test for drains that shift the tail, when inline
• 218e0bb Merge pull request #270 from servo/github-actions
• 52c50af Replace TravisCI with Github Actions.
• 5ae217a Include the cost of shifts in insert/remove benchmarks (#268)
• 58edc0e Version 1.7.0
• 1e4b151 Added feature const_new which enables SmallVec::new_const() (#265)
• Additional commits viewable in compare view

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