build(deps): Bump syn from 2.0.65 to 2.0.66 (#549)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.65 to 2.0.66.
<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>2.0.66</h2>
<ul>
<li>Allow braced structs when parsing ExprLet (<a
href="https://redirect.github.com/dtolnay/syn/issues/1671">#1671</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/b992916bdac459d279bb15098507d43b1febc50e"><code>b992916</code></a>
Release 2.0.66</li>
<li><a
href="https://github.com/dtolnay/syn/commit/4f0a23f7e147e801a996975662b0d6e6d8a7d13b"><code>4f0a23f</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1671">#1671</a>
from dtolnay/exprlet</li>
<li><a
href="https://github.com/dtolnay/syn/commit/c6d87a7aa0fda4a8ef867e833d14d105d07ca62b"><code>c6d87a7</code></a>
Allow braced structs when parsing ExprLet</li>
<li><a
href="https://github.com/dtolnay/syn/commit/747f42f235f8e1b5551a6aeca1d2779dce413408"><code>747f42f</code></a>
Update with proc-macro2 1.0.83's syntax tree sizes</li>
<li>See full diff in <a
href="https://github.com/dtolnay/syn/compare/2.0.65...2.0.66">compare
view</a></li>
</ul>
</details>
<br />


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