Implement AbsDiffEq, RelativeEq and UlpsEq from approxim crate for IndexMap

[stanpapa]

This PR adds AbsDiffEq, RelativeEq and UlpsEq from the approxim crate for IndexMap. The trait implementation are locked behind the approxim feature flag.

These traits build on top of PartialEq and are used to compare values with variable precision, which simplifies float comparisons.

[cuviper]

This seems like a quite niche thing to add, in part because approxim has much less usage than the original approx crate, yet we don't have that implementation either.

These traits also don't seem to be container-oriented -- they're not implemented for the standard BTreeMap or HashMap, nor even Vec. They are implemented for slices, but I'd guess that's more simd-oriented. What is the use case where you need an IndexMap itself to implement these traits?

Without any new dependencies, I could imagine having a generic comparison method, something like:

fn eq_by(&self, other: &IndexMap<K, V2, S2>, eq: impl FnMut(&V, &V2)) -> bool;

Would that meet your needs?