maint: create generic set and use it

[kentquirk]

Which problem is this PR solving?

• Now that Go supports some forms of generics, we can create and use a generic set rather than monkeying around with map[T]struct{}. It's clearer and just as fast.

Short description of the changes

• This adds a new type in a "generics" package. The code is mostly copied from similar code we have internally in another repository at Honeycomb.
• Same tests as in the original.
• Uses it in all but one of the places it could be used (I didn't touch the last one in sampler_config.go because it will affect work going on in a different branch; we can cherry pick this over and then fix it there.)
• Also includes de-flaking an unrelated test