Simplify syntax-path API to use integer-only indices

[Copilot]

• Inline syntax-path-element? predicate (just use exact-nonnegative-integer?)
• Fix test cases for dotted syntax - #'(a . (b c)) is treated as #'(a b c)
• Add tests for unflatten-improper-list helper function
• Replace syntax-path-element<=> with natural<=> from rebellion
• Improve flatten logic to properly handle dotted syntax
• Distinguish between dotted syntax (flattens to proper list) and truly improper lists (atom tail)
• Fix syntax-label-paths to preserve original structure while using flattened indices
• Remove syntax-path-element? definition entirely - use exact-nonnegative-integer? directly
• Remove syntax-path-element<=> definition entirely - use natural<=> directly
• Remove redundant test for natural<=> (already tested by Rebellion)
• All 904 tests pass

Original prompt

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This section details on the original issue you should resolve

<issue_title>Get rid of improper syntax paths</issue_title>
<issue_description>They're more trouble than they're worth. The syntax-path API would be much simpler if a syntax path only ever contained a list of integers, and it could be used to traverse syntax objects that contain any kind of datum. For instance, (syntax-ref #'(a b c) (syntax-path (list 0))) and (syntax-ref #'#(a b c) (syntax-path (list 0))) should both return #'a, regardless of whether the parent syntax object contains a list datum or a vector datum. This should work for all of the possible datum types except for hashes, with a caveat for improper lists:

• Box datums are treated like lists containing one element.
• Prefab struct datums are treated like lists containing their fields.
• Improper lists are treated like lists where the improper tail is the last element of the list.
• Dotted syntax is flattened, e.g. #'(a . (b . (c . d))) is treated the same as #'(a b c . d).

Hash value datums become completely unsupported. That's probably fine: hash literals cause tons of other problems anyway because their value expressions have an undefined and arbitrary ordering that's unrelated to the order they appear in source code.

This might cause various obscure breakages because syntax paths are used extensively in the core source analysis engine of Resyntax. In addition to automated tests, it would be useful to manually test this change by running Resyntax on a large project like Typed Racket or DrRacket.</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Get rid of improper syntax paths #654

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[coveralls]

Pull Request Test Coverage Report for Build #105

Details

• 138 of 178 (77.53%) changed or added relevant lines in 1 file are covered.
• 93 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.7%) to 92.871%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
private/syntax-path.rkt	138	178	77.53%

Files with Coverage Reduction	New Missed Lines	%
private/syntax-path.rkt	13	81.92%
default-recommendations/analyzers/identifier-usage.rkt	16	93.17%
private/matching-comparator.rkt	64	41.09%

Totals
Change from base Build #100:	-0.7%
Covered Lines:	14474
Relevant Lines:	15585

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💛 - Coveralls

[jackfirth]

@copilot

[jackfirth]

@copilot

[jackfirth]

@copilot