Fix doc typo in `indexBy`.
Optimize `isEqual` for primitives.
Add falsey value optimizations for `baseDifference`, `baseFlatten`, `baseUniq`, `flatten`, and `uniq`.
Cleanup the `callback` assignment in `clone` and `cloneDeep`.
Add and modify comments explaining `+0` and `-0` comparisons.
Add fast path for falsey values to the Underscore build of `flatten`.