move keyed each diffing into a shared helper

[Rich-Harris]

This is a follow-up to both #1249 that starts to make good on the promise of #1173. It moves the logic for updating a keyed each block into a shared function.

While this does mean that a standalone component with a keyed each block will become slightly larger (because the function needs to accommodate e.g. blocks with and without transitions), there are substantial benefits in return: an app with two or more keyed each blocks will shrink (in terms of filesize, startup time and memory usage), and performance should improve because the function will warm up quicker if it's called from multiple places.

Granted, this is all fairly obvious from the point of view of a traditional framework.

An equally big win, in my view, is that the codebase itself becomes somewhat simpler and easier to reason about.

[Rich-Harris]

Looks like the new algorithm breaks in some very specific cases. Looking into it...

[codecov-io]

Codecov Report

Merging #1256 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1256      +/-   ##
==========================================
- Coverage   91.89%   91.87%   -0.02%     
==========================================
  Files         126      126              
  Lines        4552     4543       -9     
  Branches     1484     1477       -7     
==========================================
- Hits         4183     4174       -9     
  Misses        153      153              
  Partials      216      216

Impacted Files	Coverage Δ
src/generators/nodes/EachBlock.ts	98.03% <ø> (-0.11%)	⬇️

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