add back the iteration based seeding for `Array`

[KristofferC]

These were removed in https://github.com/JuliaDiff/ForwardDiff.jl/pull/472 and while I was a bit uncomfortable with it I did not protest. However, now after doing some benchmarks I see that this has a runtime cost that is very much nonsignificant.

Below is a profile and the big peaks are from seed!.

If I use the iteration-based seed function they completely disappear. In order to not break anything, I added these back but with a restriction to Array so that e.g. GPU arrays will keep using the broadcast based seeding.

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.32 :tada:

Comparison is base (1592fe9) 87.28% compared to head (bafdb3b) 87.60%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #634      +/-   ##
==========================================
+ Coverage   87.28%   87.60%   +0.32%     
==========================================
  Files          10       10              
  Lines         912      936      +24     
==========================================
+ Hits          796      820      +24     
  Misses        116      116              

Impacted Files	Coverage Δ
src/apiutils.jl	100.00% <100.00%> (ø)

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[devmotion]

I wonder if one could use map! instead of broadcasting? I would assume that this would still be GPU-compatible but possibly there is (almost) no runtime cost for Array?