Add some missing Numeric[_] instances, and clean up.

We were missing instances for Byte, Short, and Real. This commit
adds them, and does a very small amount of clean up as well.

Some areas that need future attention:

  1. Better name/structure for ApproximationContext
  2. Standardizing approximation contexts for Rational
  3. Maybe we can shrink/remove ConvertableTo/ConvertableFrom?