Performance improvement with cell reuse

[lkzhao]

Regarding #97
@Banck gives us a brilliant idea of not removing the cells from CollectionView.
This brings major improvement in main thread cpu time.

With 10s scrolling session, reuseManager's lifeSpan set to 5.0s. and scrolling from top to the end every 1 second. We get the following result on an iPhone X.

Notices that CollectionView.loadCells() cpu time changed from 558ms to 288ms.

Before

After

UICollectionView

And just for comparison with UICollectionView.
UICollectionView has about the same scroll performance as CollectionKit after this patch.

[brightfuture]

Absolutely amazing improvement! 👏🏻

[rebeloper]

Can’t wait for this to be accepted! :)

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@cc63503). Click here to learn what that means.
The diff coverage is 75%.

@@           Coverage Diff            @@
##             master    #100   +/-   ##
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  Coverage          ?   56.1%           
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  Files             ?      35           
  Lines             ?     991           
  Branches          ?       0           
========================================
  Hits              ?     556           
  Misses            ?     435           
  Partials          ?       0

Impacted Files	Coverage Δ
Sources/Extensions/UIView+CollectionKit.swift	58.33% <75%> (ø)

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