ensure consistency among `Base.iterate` and `Base.in`

The definitions at

https://github.com/JuliaGeometry/GeometryBasics.jl/blob/234699e015ccb23bdaab26539dfc8fdfa9ff6add/src/basic_types.jl#L47-L49

are convenient, but they induce a non-geometric definition of Base.in, due to the fallback definition https://github.com/JuliaLang/julia/blob/4b739e711a7241f48ec0525e97c44cc870a9af99/base/operators.jl#L1109-L1120 and furthermore a geometric definition like the following

https://github.com/JuliaGeometry/GeometryTypes.jl/blob/4f1ba71d7df1341087f84974f869ea7a421eafa8/src/polygon.jl#L21-L28

are inconsistent with the documentation for Base.in. Since it is a shame to be inconsistent (because generic programs may break since they may rely on a consistency of iterate and in, and since it is a shame to lose the notation of point ∈ polygon, I propose ensuring that either

1. a type represents an infinite set of points and does not implement iterate
2. a type represents a finite set of points and implements iterate

Start with the definitions in src/basic_types.jl at the referenced lines, then compare Julia's Base.in fallback and the prior GeometryTypes.jl polygon definition. Determine how the affected geometry types should satisfy the proposed finite-set or infinite-set distinction, and confirm that iterate, in, and their documentation are consistent.