build(deps): Bump syn from 2.0.89 to 2.0.90 (#620)

Bumps [syn](https://github.com/dtolnay/syn) from 2.0.89 to 2.0.90.
<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>2.0.90</h2>
<ul>
<li>Fix automatic parenthesization of subexpressions containing outer
attributes, such as <code>(#[attr] thing).field</code> (<a
href="https://redirect.github.com/dtolnay/syn/issues/1785">#1785</a>)</li>
<li>Fix automatic parenthesization of function calls via a struct field,
such as <code>(thing.field)()</code> and <code>thing.0()</code> (<a
href="https://redirect.github.com/dtolnay/syn/issues/1786">#1786</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/syn/commit/ac5b41c852137c3d147202e3e6b2666c924cb87c"><code>ac5b41c</code></a>
Release 2.0.90</li>
<li><a
href="https://github.com/dtolnay/syn/commit/9037abbef06593189ca65f48b884a2704c2ccbb1"><code>9037abb</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1787">#1787</a>
from dtolnay/unparenthesizetest</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1a549c077232951c8f7f87d168f67a5c36b2c83f"><code>1a549c0</code></a>
Improve test_unparenthesize to catch more false negatives</li>
<li><a
href="https://github.com/dtolnay/syn/commit/f9aa20671b57932f3d61a01b4da0e99835388b1c"><code>f9aa206</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1786">#1786</a>
from dtolnay/fieldcall</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1445ccf5866dadeabcbd05f4d1bd3543298c1e2d"><code>1445ccf</code></a>
Do not parenthesize unnamed tuple struct fields in call</li>
<li><a
href="https://github.com/dtolnay/syn/commit/48d0101557047df08ca7859ab8c5eae9cb6c0805"><code>48d0101</code></a>
Fix parenthesization of field expressions in function calls</li>
<li><a
href="https://github.com/dtolnay/syn/commit/0e5c5686503d7ef41b344c2464e35e6329ed75f1"><code>0e5c568</code></a>
Add test of parentheses needed in call of field</li>
<li><a
href="https://github.com/dtolnay/syn/commit/1cf735ef91f64650f2a11e43d2ca327925b1776c"><code>1cf735e</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/syn/issues/1785">#1785</a>
from dtolnay/attrprecedence</li>
<li><a
href="https://github.com/dtolnay/syn/commit/204c1556e49c4c01f11481977d6deb9f1de36a15"><code>204c155</code></a>
Lower precedence of expressions containing outer attrs</li>
<li><a
href="https://github.com/dtolnay/syn/commit/401399fbb6225caa75b2008daaa9dc284a268076"><code>401399f</code></a>
Add test of attr precedence inside subexpression</li>
<li>Additional commits viewable in <a
href="https://github.com/dtolnay/syn/compare/2.0.89...2.0.90">compare
view</a></li>
</ul>
</details>
<br />


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