Implement specialized _findin when second arg is AbstractSet

[garrison]

Prior to this PR, _findin converts its second argument to a Set before using it. However, this overhead is unnecessary if the argument is already an AbstractSet. _findin is in turn called by findall, so this change provides a fast path for findall(in(s), ...), where s is an AbstractSet:

julia> s = Set([1,2,3,4])
Set{Int64} with 4 elements:
  4
  2
  3
  1

julia> findall(in(s), [1,3,5,7,5,3,1])
4-element Vector{Int64}:
 1
 2
 6
 7

[garrison]

I believe that the CI failures are not related to the changes in this PR.

[adienes]

seems good to me

[KristofferC]

Perhaps a rebase so it gets a fresh CI run and then merge?

[adienes]

might be an exercise in frustration given that CI has like a 10% pass rate over the past few weeks

[garrison]

Should I rebase again, or is there a better way to have the failed CI jobs run again?

[adienes]

I think close-and-reopen should trigger CI

[adienes]

I guess not :)