Class based size source

[lkzhao]

#98

[lkzhao]

This PR not only introduce class-based size source but also some convenience inits for creating BasicProvider.
We can now provide Array as DataSource, viewUpdater function as viewSource, and also provide the old sizeSource function as SizeSource.

[codecov-io]

Codecov Report

Merging #107 into master will decrease coverage by 3.81%.
The diff coverage is 17.11%.

@@            Coverage Diff             @@
##           master     #107      +/-   ##
==========================================
- Coverage    56.1%   52.28%   -3.82%     
==========================================
  Files          35       37       +2     
  Lines         991     1094     +103     
==========================================
+ Hits          556      572      +16     
- Misses        435      522      +87

Impacted Files	Coverage Δ
Sources/Provider/ComposedHeaderProvider.swift	0% <0%> (ø)	⬆️
.../Provider/ComposedHeaderProvider+Convenience.swift	0% <0%> (ø)
Sources/ViewSource/ClosureViewSource.swift	76.92% <100%> (ø)	⬆️
Sources/Provider/BasicProvider.swift	77.77% <100%> (ø)	⬆️
Sources/Provider/BasicProvider+Convenience.swift	20.77% <20.77%> (ø)

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