add numer_mut() denom_mut()

[jaamesd]

[cuviper]

At minimum, this has the same hazards as new_raw, as these could be used to create non-reduced values and/or non-positive denominators.

What is the use case? (Ideally some real case you're facing, not a hypothetical.)

[jaamesd]

I'm using this to track a bunch of performance metrics in the form of / and / in a multi agent system. These metrics are then used for heuristics/telemetry so compatibility with simple filters like WSMA/EMA is desirable as is geomeans etc. Most of these are event driven so constantl structuring/destructuring is a huge pain.