fix: address clippy warnings to improve code quality

[GarmashAlex]

Description

This PR addresses several Clippy warnings throughout the codebase to improve code quality and readability:

1. In constraint_system_ref.rs, replaced map(|x| x.clone()) with more idiomatic cloned() method
2. In instance_outliner.rs, introduced a type alias InstanceOutliningFunction<F> for the complex function type to improve code readability
3. In constraint_system.rs, replaced map().flatten() pattern with the more efficient and_then() method
4. In variable.rs, fixed non-canonical implementation of PartialOrd for Variable by using the canonical pattern that leverages the already implemented Ord trait
5. In snark/src/lib.rs, introduced a type alias IndexResult for the complex return type in the UniversalSetupSNARK trait

These changes make the code more idiomatic, more readable, and follow Rust best practices. The changes are purely refactoring and don't affect functionality.

closes: #XXXX

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• Targeted PR against correct branch (master)
• Linked to Github issue with discussion and accepted design OR have an explanation in the PR that describes this work.
• Wrote unit tests
• Updated relevant documentation in the code
• Added a relevant changelog entry to the Pending section in CHANGELOG.md
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