ensure consistency among `Base.iterate` and `Base.in` about geometrybasics.jl

[goretkin]

In case this example is helpful:

julia> Base.IteratorSize(::Type{<:GeometryBasics.Polygon}) = Base.SizeUnknown() # or define `length(::Polygon)` This is required by the [iterator interface](https://docs.julialang.org/en/v1/manual/interfaces/#man-interface-iteration)

julia> p = Polygon([Point(0,0), Point(0, 1), Point(1, 0)])
Polygon{2,Int64,Point{2,Int64},LineString{2,Int64,Point{2,Int64},Base.ReinterpretArray{GeometryBasics.Ngon{2,Int64,2,Point{2,Int64}},1,Tuple{Point{2,Int64},Point{2,Int64}},TupleView{Tuple{Point{2,Int64},Point{2,Int64}},2,1,Array{Point{2,Int64},1}}}},Array{LineString{2,Int64,Point{2,Int64},Base.ReinterpretArray{GeometryBasics.Ngon{2,Int64,2,Point{2,Int64}},1,Tuple{Point{2,Int64},Point{2,Int64}},TupleView{Tuple{Point{2,Int64},Point{2,Int64}},2,1,Array{Point{2,Int64},1}}}},1}}(GeometryBasics.Ngon{2,Int64,2,Point{2,Int64}}[Line([0, 0] => [0, 1]), Line([0, 1] => [1, 0])], LineString{2,Int64,Point{2,Int64},Base.ReinterpretArray{GeometryBasics.Ngon{2,Int64,2,Point{2,Int64}},1,Tuple{Point{2,Int64},Point{2,Int64}},TupleView{Tuple{Point{2,Int64},Point{2,Int64}},2,1,Array{Point{2,Int64},1}}}}[])

julia> Point(.2,.2) in p
false

julia> Point(0,0) in p
true

julia> (0,0) in p
false

julia> [0,0] in p
true