title: fungible fixes and more conformance tests

doc:
  - audience: Runtime Dev
    description: |
      Adds conformance tests for the Balanced and Unbalanced fungible traits
      Fixes Unbalanced::decrease_balance not respecting preservation
      Fixes Balanced::pair possibly returning pairs of imbalances which do not cancel each other out. Method now returns a Result instead (breaking change).
      Fixes Balances pallet active_issuance possible 'underflow'
      Refactors the conformance test file structure to match the fungible file structure: tests for traits in regular.rs go into a test file named regular.rs, tests for traits in freezes.rs go into a test file named freezes.rs, etc.
      Improve doc comments
      Simplify macros

crates:
  - name: pallet-balances
  - name: frame-support