build(deps): Bump proc-macro2 from 1.0.89 to 1.0.91 (#611)

Bumps [proc-macro2](https://github.com/dtolnay/proc-macro2) from 1.0.89
to 1.0.91.
<details>
<summary>Release notes</summary>
<p><em>Sourced from <a
href="https://github.com/dtolnay/proc-macro2/releases">proc-macro2's
releases</a>.</em></p>
<blockquote>
<h2>1.0.91</h2>
<ul>
<li>Fix panic <em>&quot;compiler/fallback mismatch 949&quot;</em> when
using TokenStream::from_str from inside a proc macro to parse a string
containing doc comment (<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/484">#484</a>)</li>
</ul>
<h2>1.0.90</h2>
<ul>
<li>Improve error recovery in TokenStream's and Literal's FromStr
implementations to work around <a
href="https://redirect.github.com/rust-lang/rust/issues/58736">rust-lang/rust#58736</a>
such that rustc does not poison compilation on codepaths that should be
recoverable errors (<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/477">#477</a>,
<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/478">#478</a>,
<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/479">#479</a>,
<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/480">#480</a>,
<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/481">#481</a>,
<a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/482">#482</a>)</li>
</ul>
</blockquote>
</details>
<details>
<summary>Commits</summary>
<ul>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/5046761b9689109142a02c71736284013673f0bd"><code>5046761</code></a>
Release 1.0.91</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/27c54948b3c993855f2a58decc154ebe0a560f66"><code>27c5494</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/486">#486</a>
from dtolnay/compilerlex</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/a9146d6cb558851b1ae180ef28b93cd6987cd4ab"><code>a9146d6</code></a>
Ensure that compiler tokenstream parsing only produces a compiler
lexerror</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/1ce5f04a7e44e2213244da36767678d53567c4f7"><code>1ce5f04</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/485">#485</a>
from dtolnay/fallbackident</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/75d08189223012a175e7165cd665943dc7f5ee9f"><code>75d0818</code></a>
Make parser's fallback Ident symmetric with Group and Literal</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/56c3e31df20a7c5c6ab9d356332a6a51f78ba179"><code>56c3e31</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/484">#484</a>
from dtolnay/fbliteral</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/d2c0e611fdea620dd85f8ab8327cdccb0f002cbb"><code>d2c0e61</code></a>
Fix spanned fallback literal construction</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/ec556456634f42cb1f97ca2788dd20ac6b29f48c"><code>ec55645</code></a>
Release 1.0.90</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/87a180c12fad911a6175e047ae9faa979ae73376"><code>87a180c</code></a>
Merge pull request <a
href="https://redirect.github.com/dtolnay/proc-macro2/issues/482">#482</a>
from dtolnay/validate</li>
<li><a
href="https://github.com/dtolnay/proc-macro2/commit/da66636bc24f3f4d45e814a4303a3082c44f1a94"><code>da66636</code></a>
Perform up-front validation using the fallback parser</li>
<li>Additional commits viewable in <a
href="https://github.com/dtolnay/proc-macro2/compare/1.0.89...1.0.91">compare
view</a></li>
</ul>
</details>
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