IteratorSize for a StructArray

[jishnub]

This makes the following fail instead of running out of memory:

julia> using StructArrays, InfiniteArrays

julia> S = StructArray{Complex{Int}}((1:∞, 1:∞));

julia> collect(i for i in S)
ERROR: MethodError: no method matching _similar_shape(::Base.Generator{StructVector{Complex{Int64}, NamedTuple{(:re, :im), Tuple{InfiniteArrays.InfUnitRange{Int64}, InfiniteArrays.InfUnitRange{Int64}}}, Int64}, typeof(identity)}, ::Base.IsInfinite)

[piever]

Thanks, looks good! Happy to merge after a rebase (sorry, might have created some conflicts on the Project file with the sparse arrays PR).