Update to stable smallvec

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servo/rust-smallvec#175

[8573]

Stable smallvec's minimum supported Rust version (MSRV) is higher than irc-bot's MSRV, so updating to stable smallvec won't proceed until there comes a bump in irc-bot's MSRV. However, when that time comes, I also would consider switching to tinyvec, which is written in Safe Rust and doesn't have a history of safety problems.