`avg_` and `sum_` should require `Num` instances

Like `round_`, `floor_` and `ceiling_`, `avg_` and `sum_` should have `Num` instances because their SQL function equivalents can only be used on numbers.

As any program using this functions on non-numeric values would crash already, this may even be possible to do in a minor version bump (but I understand unwillingness to do so).