Improve datasource caching

[benasher44]

Changes in this pull request

• Cache view registration info across diffs
• Cache startIndex and endIndex to avoid re-querying original data source for count

Checklist

• [x] All tests pass. Demo project builds and runs.
• [x] I added tests, an experiment, or detailed why my change isn't tested.
• [x] I added an entry to the CHANGELOG.md for any breaking changes, enhancements, or bug fixes.
• [x] I have reviewed the contributing guide

[codecov-commenter]

Codecov Report

Merging #190 into master will decrease coverage by 0.05%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #190      +/-   ##
==========================================
- Coverage   80.39%   80.34%   -0.05%     
==========================================
  Files          33       33              
  Lines        2233     2238       +5     
==========================================
+ Hits         1795     1798       +3     
- Misses        438      440       +2     

Impacted Files	Coverage Δ
Sources/Diffing.swift	80.00% <100.00%> (-1.11%)	:arrow_down:
Sources/TableCellViewModelDataSource.swift	78.26% <100.00%> (ø)