cleanup simple view provider

[lkzhao]

Now that class-based size source is merged. we can now simplify SimpleViewProvider.
In this PR

• introduces a new SimpleViewSizeSource class that is responsible for taking care of view sizing.
• makes SimpleViewProvider a subclass BasicProvider again.
• adds some tests for BasicProvider and WaterfallLayout.
• general cleanup after class-based size source PR.

[codecov-io]

Codecov Report

❗ No coverage uploaded for pull request base (master@b704b3b). Click here to learn what that means.
The diff coverage is 23.25%.

@@            Coverage Diff            @@
##             master     #108   +/-   ##
=========================================
  Coverage          ?   70.16%           
=========================================
  Files             ?       41           
  Lines             ?     1123           
  Branches          ?        0           
=========================================
  Hits              ?      788           
  Misses            ?      335           
  Partials          ?        0

Impacted Files	Coverage Δ
Sources/Protocol/Provider.swift	100% <ø> (ø)
Sources/DataSource/ClosureDataSource.swift	0% <0%> (ø)
Sources/SizeSource/UIImageSizeSource.swift	0% <0%> (ø)
Sources/SizeSource/SimpleViewSizeSource.swift	0% <0%> (ø)
Sources/ViewSource/ViewSource.swift	50% <0%> (ø)
Sources/SizeSource/ClosureSizeSource.swift	100% <100%> (ø)
Sources/Provider/BasicProvider.swift	77.77% <100%> (ø)
Sources/DataSource/ArrayDataSource.swift	100% <100%> (ø)
Sources/Provider/ComposedHeaderProvider.swift	78.65% <100%> (ø)
Sources/SizeSource/SizeSource.swift	33.33% <33.33%> (ø)

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