refactor

[Rich-Harris]

See #1316.

Lots more work to do here, but I'm optimistic about the direction it's going in. Essentially the goals are:

• speed up compilation, by
  • reducing the number of times the compiler has to walk the tree
  • not changing prototypes of objects after they've been created
  • being more disciplined about maintaining object shapes
  • not needing to clone the AST
• DRY out certain bits of code
• make the codebase slightly easier to dive into, particularly by getting rid of all the bewildering contextualise stuff
• stretch goal: unify DOM and SSR compilers

I found myself leaning towards a couple of naming changes — internally, generator is referred to more simply as compiler, and in the generated code, state becomes ctx, since it's more appropriate to speak of a 'context' once you're inside each/await blocks.

[codecov-io]

Codecov Report

Merging #1367 into master will decrease coverage by 0.38%.
The diff coverage is 92.33%.

@@            Coverage Diff             @@
##           master    #1367      +/-   ##
==========================================
- Coverage   91.49%   91.11%   -0.39%     
==========================================
  Files         126      120       -6     
  Lines        4469     4421      -48     
  Branches     1384     1360      -24     
==========================================
- Hits         4089     4028      -61     
- Misses        154      156       +2     
- Partials      226      237      +11

Impacted Files	Coverage Δ
src/utils/flattenReference.ts	71.42% <0%> (-5.5%)	⬇️
src/validate/html/index.ts	97.95% <100%> (+0.08%)	⬆️
src/compile/nodes/Head.ts	100% <100%> (ø)
src/compile/dom/Block.ts	95.83% <100%> (ø)
src/compile/nodes/PendingBlock.ts	100% <100%> (ø)
src/compile/nodes/Binding.ts	96.03% <100%> (ø)
src/compile/nodes/EventHandler.ts	100% <100%> (ø)
src/compile/nodes/Comment.ts	100% <100%> (ø)
src/compile/nodes/ElseBlock.ts	100% <100%> (ø)
src/compile/nodes/shared/TemplateScope.ts	100% <100%> (ø)
... and 59 more

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[Rich-Harris]

At long last, the diff is now more red than green. Will probably find a few more things to tidy up but I reckon this is basically ready — going to remove the [WIP] label.

Most apps should get slightly smaller as a result of this PR, due to the way context is kept up to date.

[Rich-Harris]

@Conduitry ah, good catch, thanks