Auto merge of #26577 - servo:dependabot/cargo/slab-0.4.2, r=jdm

Bump slab from 0.4.1 to 0.4.2

Bumps [slab](https://github.com/carllerche/slab) from 0.4.1 to 0.4.2.
<details>
<summary>Changelog</summary>
<p><em>Sourced from <a href="https://github.com/carllerche/slab/blob/master/CHANGELOG.md">slab's changelog</a>.</em></p>
<blockquote>
<h1>0.4.2 (January 11, 2019)</h1>
<ul>
<li>Add <code>Slab::drain</code> (<a href="https://github-redirect.dependabot.com/carllerche/slab/issues/56">#56</a>).</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a href="e6b8676a15"><code>e6b8676</code></a> Old rust versions only have build</li>
<li><a href="128eb298b4"><code>128eb29</code></a> Only <code>check</code> on older Rust versions</li>
<li><a href="9a327e7092"><code>9a327e7</code></a> Fix cargo fmt</li>
<li><a href="0bcfd5344d"><code>0bcfd53</code></a> Fix CI</li>
<li><a href="7802cb9460"><code>7802cb9</code></a> Bump version to v0.4.2</li>
<li><a href="a0c2d19c67"><code>a0c2d19</code></a> Deny unreachable-pub</li>
<li><a href="9c114f0ca0"><code>9c114f0</code></a> Implement drain iterator (<a href="https://github-redirect.dependabot.com/carllerche/slab/issues/56">#56</a>)</li>
<li><a href="208c40c59e"><code>208c40c</code></a> Apply rustfmt 1.0.0 (<a href="https://github-redirect.dependabot.com/carllerche/slab/issues/57">#57</a>)</li>
<li>See full diff in <a href="https://github.com/carllerche/slab/compare/v0.4.1...v0.4.2">compare view</a></li>
</ul>
</details>
<br />

[![Dependabot compatibility score](https://api.dependabot.com/badges/compatibility_score?dependency-name=slab&package-manager=cargo&previous-version=0.4.1&new-version=0.4.2)](https://dependabot.com/compatibility-score/?dependency-name=slab&package-manager=cargo&previous-version=0.4.1&new-version=0.4.2)

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[//]: # (dependabot-automerge-start)
[//]: # (dependabot-automerge-end)

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Cargo.lock

@@ -5323,9 +5323,9 @@ version = "0.0.1"
 
 [[package]]
 name = "slab"
-version = "0.4.1"
+version = "0.4.2"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "5f9776d6b986f77b35c6cf846c11ad986ff128fe0b2b63a3628e3755e8d3102d"
+checksum = "c111b5bd5695e56cffe5129854aa230b39c93a305372fdbb2668ca2394eea9f8"
 
 [[package]]
 name = "smallbitvec"