`compare_asset_class`: camelCase to snake_case

[ignaciodopazo]

https://github.com/SundaeSwap-finance/sundae-contracts/blob/main/validators/pool.ak#L556-L560

Replace aPolicy with a_policy and bPolicy with b_policy for consistency with the variable definitions in the rest of the codebase, and vscode's Aiken extension highlighting is weird when using camelCase for variables as well.