Base: intersect keep the container type

[barucden]

A bug was introduced for 3 arguments version of intersect in #41769. The container type always changed to Set. See this comment.

master:

julia> intersect(BitSet([1,2]), [1,2], [2])
Set{Int64} with 1 element:
  2

After this PR:

julia> intersect(BitSet([1,2]), [1,2], [2])
BitSet with 1 element:
  2

julia> intersect(BitSet([1,2]), [1,2], [2.0])
Set{Float64} with 1 element:
  2.0

I kindly ask you to suggest improvements as this PR introduces a slow-down (compared to 1.6.2):

# 1.6.2
julia> @btime intersect(BitSet([1,2]), [1,2], [2])
  397.583 ns (12 allocations: 768 bytes)
BitSet with 1 element:
  2
# PR
julia> @btime intersect(BitSet([1,2]), [1,2], [2])
  516.840 ns (15 allocations: 928 bytes)
BitSet with 1 element:
  2

EDIT: cc @vtjnash who first noticed this bug

[JeffBezanson]

Maybe we could get the performance back by skipping the copy when T == promote_eltype(s, itr)?

[barucden]

Thanks @JeffBezanson! Now the timings are the same:

# this PR
julia> @btime intersect(BitSet([1,2]), [1,2], [2])
  384.952 ns (12 allocations: 752 bytes)
BitSet with 1 element:
  2
# 1.6.3
julia> @btime intersect(BitSet([1,2]), [1,2], [2])
  385.292 ns (12 allocations: 768 bytes)
BitSet with 1 element:
  2

[barucden]

Is more work required here, or can we merge it?