[WIP] Simplify everything

[Rich-Harris]

See #1122. Lots still to do, and this branch is currently rather messy, but I'm starting to think this will be a big win.

[Rich-Harris]

Tests passing. Now to tidy up the mess I just made

[codecov-io]

Codecov Report

Merging #1173 into master will decrease coverage by 0.02%.
The diff coverage is 94.64%.

@@            Coverage Diff             @@
##           master    #1173      +/-   ##
==========================================
- Coverage   91.75%   91.73%   -0.03%     
==========================================
  Files         127      127              
  Lines        4572     4560      -12     
  Branches     1495     1494       -1     
==========================================
- Hits         4195     4183      -12     
- Misses        159      160       +1     
+ Partials      218      217       -1

Impacted Files	Coverage Δ
src/generators/nodes/Fragment.ts	100% <ø> (ø)	⬆️
src/generators/nodes/Element.ts	95.12% <ø> (ø)	⬆️
...rators/server-side-rendering/visitors/EachBlock.ts	100% <100%> (ø)	⬆️
src/generators/nodes/Component.ts	95.65% <100%> (+0.43%)	⬆️
src/generators/dom/Block.ts	96.8% <100%> (+0.37%)	⬆️
src/generators/nodes/IfBlock.ts	96.52% <100%> (-0.03%)	⬇️
src/generators/dom/index.ts	96.25% <100%> (ø)	⬆️
src/generators/nodes/EachBlock.ts	98.73% <100%> (-0.03%)	⬇️
src/generators/Generator.ts	94.14% <50%> (ø)	⬆️
src/generators/nodes/AwaitBlock.ts	91.66% <75%> (-2.62%)	⬇️

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[Rich-Harris]

Think this is probably good to squash and merge. Some follow-up work:

• Only track contextual values (i.e. var thing = state.thing, and the equivalent statement inside the update method) in blocks that use them. Indexes only ever need to be updated for keyed each blocks
• See if we can now move more work (e.g. diffing keyed each blocks, handling promises etc) into shared helpers, to reduce duplication