Add cached iterator

[pepijndevos]

For iterators that are expensive to compute and cheap to store, you might want to cache the result so the next time you iterate it just returns items from the cache. In a way this is kind of like lazy sequences from Clojure/Lazy.jl

This is missing documentation and tests but would like to gauge interest before committing time to that.

[pepijndevos]

Ehm... CI is running on Julia 0.7 and 1.0 neither of which allow const struct members on mutable structs.

[codecov-commenter]

Codecov Report

Merging #103 (4e3f858) into master (3cc0482) will increase coverage by 0.54%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
+ Coverage   90.41%   90.96%   +0.54%     
==========================================
  Files           1        1              
  Lines         334      354      +20     
==========================================
+ Hits          302      322      +20     
  Misses         32       32              

Impacted Files	Coverage Δ
src/IterTools.jl	90.96% <100.00%> (+0.54%)	:arrow_up:

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[oxinabox]

Merging this presently

[oxinabox]

Hmm i have 2 more commits locally that haven't pushed, actually not merging this til I worked out why they haven't

[oxinabox]

Ah that worked to make them show up.