build(deps): Bump syn from 2.0.63 to 2.0.64 (#545)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.63 to 2.0.64.
<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>2.0.64</h2>
<ul>
<li>Support using ParseBuffer across <code>catch_unwind</code> (<a
href="https://redirect.github.com/dtolnay/syn/issues/1646">#1646</a>)</li>
<li>Validate that the expression in a let-else ends in brace as required
by rustc (<a
href="https://redirect.github.com/dtolnay/syn/issues/1648">#1648</a>, <a
href="https://redirect.github.com/dtolnay/syn/issues/1649">#1649</a>)</li>
<li>Legalize invalid const generic arguments by wrapping in braces (<a
href="https://redirect.github.com/dtolnay/syn/issues/1654">#1654</a>, <a
href="https://redirect.github.com/dtolnay/syn/issues/1655">#1655</a>)</li>
<li>Fix some expression precedence edge cases involving
<code>break</code> and <code>return</code> in loop headers (<a
href="https://redirect.github.com/dtolnay/syn/issues/1656">#1656</a>)</li>
<li>Always print closure bodies with a brace when the closure has an
explicit return type (<a
href="https://redirect.github.com/dtolnay/syn/issues/1658">#1658</a>)</li>
<li>Automatically insert necessary parentheses in ToTokens for Expr when
required by expression precedence (<a
href="https://redirect.github.com/dtolnay/syn/issues/1659">#1659</a>)</li>
<li>Support struct literal syntax in match guard expressions (<a
href="https://redirect.github.com/dtolnay/syn/issues/1662">#1662</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/98a62cbee4f3364787a4a04a9c18f2831ebb7e0a"><code>98a62cb</code></a>
Release 2.0.64</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1f8eddbf4fa8b6536c4a938ed9fef17871d1e42a"><code>1f8eddb</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1662">#1662</a>
from dtolnay/guardstruct</li>
<li><a
href="https://github.com/dtolnay/syn/commit/0a3d3bd837905bc48c6c2082f92cb7cbb2788d0d"><code>0a3d3bd</code></a>
Fix parsing of struct literal in match guard expression</li>
<li><a
href="https://github.com/dtolnay/syn/commit/f4641931718f41ab0a81c173d8e4fc1f38fe772e"><code>f464193</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1659">#1659</a>
from dtolnay/needspar</li>
<li><a
href="https://github.com/dtolnay/syn/commit/afa1c7259aa6ef5795e922f7820d95fc205a74f9"><code>afa1c72</code></a>
Add more auto-parenthesization tests</li>
<li><a
href="https://github.com/dtolnay/syn/commit/532d3a37cf33ef13e5a878a9efdde2415b4cd2fe"><code>532d3a3</code></a>
Insert necessary parentheses in ToTokens for Expr</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1b6a45079d2cfde0731ea47a23f5713f47d88d73"><code>1b6a450</code></a>
Add test of parenthesis insertion by Expr's ToTokens</li>
<li><a
href="https://github.com/dtolnay/syn/commit/83ea28918bcac980b7382c64ffe8982c70a01a18"><code>83ea289</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1658">#1658</a>
from dtolnay/closurebrace</li>
<li><a
href="https://github.com/dtolnay/syn/commit/206d897ed8dd784d7a36e7ca3b0920052c025cbc"><code>206d897</code></a>
Create braces around closure body during printing if needed</li>
<li><a
href="https://github.com/dtolnay/syn/commit/9b60027c1605420778f41a1082ec80fd730b0c25"><code>9b60027</code></a>
Ignore struct_excessive_bools pedantic clippy lint</li>
<li>Additional commits viewable in <a
href="https://github.com/dtolnay/syn/compare/2.0.63...2.0.64">compare
view</a></li>
</ul>
</details>
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