Improve XCTAssertUnorderedEqualSequences complexity to O(n) for Hashable elements

[shubhransh-gupta]

Description

Resolves #176.

Improves the algorithmic complexity of expectUnorderedEqualSequences from $O(n^2)$ to $O(n)$ when sequence elements conform to Hashable, by tracking counts with a frequency dictionary instead of sequential firstIndex(of:) removals from an Array.

Detailed Design

• Added a generic overload of expectUnorderedEqualSequences constrained to S1.Element: Hashable.
• Retained the existing S1.Element: Equatable implementation as a fallback for types that only conform to Equatable.

Checklist

• I've added at least one test that validates that my change is working, if appropriate
• I've followed the code style of the rest of the project
• I've read the Contribution Guidelines
• I've updated the documentation if necessary