Replace sketch-specific value traits with num-traits

[notfilippo]

We currently define sketch-specific value traits (DensityValue from #62, CountMinValue) that expose only the operations needed by each sketch. There is growing overlap between them.

For example, DensityValue is implemented for f32/f64, and we sometimes convert to f64 for operations.

Proposal

Evaluate replacing these traits with appropriate traits from num-traits (e.g., Float) to:

• Reduce duplication
• Reuse well-known numeric abstractions
• Avoid unnecessary conversions

This is exploratory.

[tisonkun]

I trust num-traits and would be glad to review a patch demostrate the result by just using it :D