`@0x/contracts-staking`: Denominate pool operator shares in parts-per-million.
`@0x/contracts-staking`: Update tests for new stake computation and higher precision math.
`@0x/contracts-staking`: Add `setCobbDouglasAlpha()` function.
`@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.