Use scientific >= 0.3

scientific-0.3 exports the Text and ByteString Scientific Builders from
the following modules respectively:

- Data.Text.Lazy.Builder.Scientific
- Data.ByteString.Builder.Scientific
Use floor on Scientifics again

scientific-0.3 provides a DoS safe floor.
Use Scientific.fromRealFloat and Scientific.toRealFloat
Renamed fromRealFloat back to fromFloatDigits

I think the latter is more descriptive.
Use Scientific.toRealFloat instead of realToFrac in scientificToNumber
Add warning about space-usage on the Fixed instance
Merge