Add == for Zeros-backed CachedArrays

[DanielVandH]

Fixes #327

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.15%. Comparing base (8fc085d) to head (0f02fa3).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #328      +/-   ##
==========================================
+ Coverage   94.14%   94.15%   +0.01%     
==========================================
  Files          17       17              
  Lines        3091     3097       +6     
==========================================
+ Hits         2910     2916       +6     
  Misses        181      181              

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[dlfivefifty]

To fix this properly we want to add an equals_layout to ArrayLayouts.jl a la

https://github.com/JuliaApproximation/QuasiArrays.jl/blob/d9ce29e4cb188ec5b246aaae6242589a11d873c5/src/abstractquasiarray.jl#L506

Then we can compare any AbstractPaddedLayout. But we can merge this as is for now