Bump syn from 2.0.43 to 2.0.44 (#2263)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.43 to 2.0.44.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/dtolnay/syn/releases">syn's
releases</a>.</em></p>
<blockquote>
<h2>2.0.44</h2>
<ul>
<li>Documentation improvements</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/58b42f526425b320607c4c4215e33c0e22e51acd"><code>58b42f5</code></a>
Release 2.0.44</li>
<li><a
href="https://github.com/dtolnay/syn/commit/45234377605550352f95f9b098008cab72fc57ac"><code>4523437</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1569">#1569</a>
from dtolnay/cfgvisit</li>
<li><a
href="https://github.com/dtolnay/syn/commit/694a1bb661cf8b05d2250ef921fa1f1206b37195"><code>694a1bb</code></a>
Render doc cfg on Visit/VisitMut/Fold trait methods</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1728630caa225e51636fa60f1778e446af12b59a"><code>1728630</code></a>
Add doc cfg on Error::new_spanned</li>
<li><a
href="https://github.com/dtolnay/syn/commit/649e4266ae0d3ad925b0bdb5d9ee7e6b45c2f221"><code>649e426</code></a>
Mark exprs which are not parsed in &quot;derive&quot; mode as
&quot;full&quot;-only</li>
<li><a
href="https://github.com/dtolnay/syn/commit/6c4627f24ae843b9e5404ada30cc77deb8456f02"><code>6c4627f</code></a>
Fill in missing doc cfg on Expr and Pat nodes</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1cea0bef119a07553a98a10eb144e6a95d9f343b"><code>1cea0be</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1568">#1568</a>
from dtolnay/doccfg</li>
<li><a
href="https://github.com/dtolnay/syn/commit/dc2153d04eab38c0cc180eb42e991ee63871d46c"><code>dc2153d</code></a>
Restore doc cfg on re-exports</li>
<li><a
href="https://github.com/dtolnay/syn/commit/e2b6ebc55b34240f907e6e9aa44536223563ade4"><code>e2b6ebc</code></a>
Fix typo in ast_enum cfg</li>
<li><a
href="https://github.com/dtolnay/syn/commit/a193361b83816beb63960cca7dcb511818f423fe"><code>a193361</code></a>
Fix unused_macros warning on ast_enum when features are disabled</li>
<li>Additional commits viewable in <a
href="https://github.com/dtolnay/syn/compare/2.0.43...2.0.44">compare
view</a></li>
</ul>
</details>
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