Bump the core group with 1 update

[dependabot[bot]]

Bumps the core group with 1 update: syn.

Updates syn from 2.0.39 to 2.0.40

Release notes

Sourced from syn's releases.

2.0.40

• Fix some edge cases of handling None-delimited groups in expression parser (#1539, #1541, #1542, #1543, #1544, #1545)

Commits

• cf7f40a Release 2.0.40
• 1ce8ccf Merge pull request #1538 from dtolnay/testinvisible
• d06bff8 Add test for parsing Delimiter::None in expressions
• 9ec66d4 Merge pull request #1545 from dtolnay/groupedlet
• 384621a Fix None-delimited let expression in stmt position
• 5325b6d Add test of let expr surrounded in None-delimited group
• 0ddfc27 Merge pull request #1544 from dtolnay/nonedelimloop
• 9c99b3f Fix stmt boundary after None-delimited group containing loop
• 2781584 Add test of None-delimited group containing loop in match arm
• d332928 Simplify token stream construction in Delimiter::None tests
• Additional commits viewable in compare view

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