refactor: arithmetic_side_effects: replace string-based type comparison with DefId/PrimTy lookup

[Tiauma]

changelog: [arithmetic_side_effects]: replace string-based type comparison with DefId/PrimTy lookup

Previously, the lint compared types using their string representations,
which was inefficient and failed to recognize type aliases (e.g.,
core→std aliases). This PR:

• Introduces GeneralTy enum to represent types as either PrimTy or DefId
• Builds lookup maps at initialization using paths::lookup_path_str and local crate search
• Properly resolves type aliases and paths across crates
• Removes FxHashMap in favor of UnordMap/UnordSet for deterministic output
• Preserves all existing functionality and passes tests

Fixes rust-lang/rust-clippy#16359

[rustbot]

r? @llogiq

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The reviewer was selected based on:

• Owners of files modified in this PR: 7 candidates
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• Random selection from Jarcho, dswij, llogiq, samueltardieu

[Tiauma]

r? samueltardieu