`@0x/contracts-staking`: Add `_invert()` and `_mulDiv()` to `LibFixedMath`. `@0x/contracts-staking`: Update `MixinExchangeFees._cobbDouglas()` to work with `LibFixedMath`. `@0x/contracts-staking`: Add unit and fuzz tests for `_cobbDouglas()` and remaining `LibFixedMath` functions.