`@0x/contracts-staking`: Merge `exchange_fees` unit tests into `protocol_fees` unit tests.
`@0x/contracts-staking`: Remove `ProtocolFeeActor` and any use of it.
`@0x/contracts-staking`: Remove unused constants.
`@0x/contracts-staking`: Move WETH assertion constructor into `MixinDeploymentConstants`.
`@0x/contracts-staking`: Add more unit tests.
`@0x/contracts-staking`: Move params storage variables into `MixinStorage`.
`@0x/contracts-staking`: Add storage layout tests for new state vars.
`@0x/contracts-staking`: Add more migration tests.
`@0x/contracts-staking`: Add `_initMixinParams()` function that sets up `MixinParams` state.