Refactoring: Merge the IN expression variants into Expr::In

[LucaCappelletti94]

Expr had three variants for the IN predicate, InList, InSubquery and InUnnest, each repeating the left-hand expression and the NOT flag, two of them repeating ClickHouse's GLOBAL flag, and each with its own copy of the Display, Spanned and parser code. Supporting another right-hand side, such as SQLite's IN table_name, meant a fourth copy.

They are now one Expr::In { expr, target, negated, global }, where target is an InTarget: List, Subquery or Unnest. The SQL that parses and the text printed back are unchanged, GLOBAL is still only accepted before a list or subquery, and size_of::<Expr>() stays at 328 bytes. This is a breaking change for code that builds or matches Expr::InList, Expr::InSubquery or Expr::InUnnest, which now match Expr::In with the corresponding InTarget.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.57143% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 81.30%. Comparing base (d32825d) to head (a16fbaf).

Files with missing lines	Patch %	Lines
src/ast/spans.rs	0.00%	6 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2679   +/-   ##
=======================================
  Coverage   81.29%   81.30%           
=======================================
  Files          42       42           
  Lines       33950    33944    -6     
  Branches    33950    33944    -6     
=======================================
- Hits        27601    27598    -3     
+ Misses       2785     2782    -3     
  Partials     3564     3564           

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