IteratorSize for infinite ranges

[jishnub]

This lets one use the ranges as iterators, and the following works after this:

julia> s = Iterators.Stateful(2:∞)
Base.Iterators.Stateful{InfiniteArrays.InfUnitRange{Int64}, Tuple{Int64, Int64}, Int64}(2:∞, (2, 2), -1)

julia> first(s)
2

julia> first(s)
3

[codecov]

Codecov Report

Merging #105 (83d6f39) into master (0873436) will decrease coverage by 0.03%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #105      +/-   ##
==========================================
- Coverage   85.03%   85.01%   -0.03%     
==========================================
  Files           4        4              
  Lines         655      654       -1     
==========================================
- Hits          557      556       -1     
  Misses         98       98              

Impacted Files	Coverage Δ
src/infrange.jl	87.86% <100.00%> (+0.03%)	⬆️
src/InfiniteArrays.jl	86.51% <0.00%> (-0.44%)	⬇️
src/infarrays.jl	82.54% <0.00%> (+0.08%)	⬆️

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