Split Linear.Epsilon into own package.

I find often needing it when doing something numeric (e.g. `ad`), but depending on `linear` feels wrong. A package (`epsilon`) would be very nice. I'll probably will make it anyway, but it would be nice to dedup functionality on Hackage. Another example is ApproxEq in `aeson` which does slightly different thing, but could reuse `Epsilon` class for implementations. EDIT: also `half` could depend on it.