chore(deps): Bump serde from 1.0.208 to 1.0.209 (#1484)

Bumps [serde](https://github.com/serde-rs/serde) from 1.0.208 to
1.0.209.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/serde-rs/serde/releases">serde's
releases</a>.</em></p>
<blockquote>
<h2>v1.0.209</h2>
<ul>
<li>Fix deserialization of empty structs and empty tuples inside of
untagged enums (<a
href="https://github.com/serde-rs/serde/issues/2805">#2805</a>,
thanks <a
href="https://github.com/Mingun"><code>@​Mingun</code></a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/serde-rs/serde/commit/30752ac4ffdaa284606eda34055ad185e28c5499"><code>30752ac</code></a>
Release 1.0.209</li>
<li><a
href="https://github.com/serde-rs/serde/commit/b84e6ca4f5fef69b3de985c586a07b1246f3eb9a"><code>b84e6ca</code></a>
Improve wording of PR 2805 comments</li>
<li><a
href="https://github.com/serde-rs/serde/commit/87a2fb0f1a2774ea5bb20c0ed988b9ba57fc8166"><code>87a2fb0</code></a>
Wrap comments from PR 2805 to 80 columns</li>
<li><a
href="https://github.com/serde-rs/serde/commit/9eaf7b9824f2082c50d17ad22b786322dc283a61"><code>9eaf7b9</code></a>
Merge pull request <a
href="https://github.com/serde-rs/serde/issues/2805">#2805</a>
from Mingun/untagged-tests</li>
<li><a
href="https://github.com/serde-rs/serde/commit/7bde100237875d4f435de5ad90074b0479c37486"><code>7bde100</code></a>
Replace MapRefDeserializer with value::MapDeserializer</li>
<li><a
href="https://github.com/serde-rs/serde/commit/da7fc795ee654252effa232a62a5a1e6d4f551ee"><code>da7fc79</code></a>
Fix deserialization of empty struct variant in untagged enums</li>
<li><a
href="https://github.com/serde-rs/serde/commit/4c5fec1363d363f995375426f72db11c28f357c1"><code>4c5fec1</code></a>
Test special cases that reaches SeqRefDeserializer::deserialize_any
len==0 co...</li>
<li><a
href="https://github.com/serde-rs/serde/commit/6588b0ad3777f7ad930d68ab4b9ec5b9c25398e0"><code>6588b0a</code></a>
Cover Content::Seq case in VariantRefDeserializer::struct_variant</li>
<li><a
href="https://github.com/serde-rs/serde/commit/0093f74cfee5ee3239514a7aad5fb44843eddcdd"><code>0093f74</code></a>
Split test newtype_enum into four tests for each variant</li>
<li><a
href="https://github.com/serde-rs/serde/commit/171c6da57af712cfcf01c6c124b14cabfca364ba"><code>171c6da</code></a>
Complete coverage of
ContentRefDeserializer::deserialize_newtype_struct</li>
<li>Additional commits viewable in <a
href="https://github.com/serde-rs/serde/compare/v1.0.208...v1.0.209">compare
view</a></li>
</ul>
</details>
<br />


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