[WIP] Unmount nodes in destroy methods

[Rich-Harris]

block.u() is always followed by block.d(). The reason for the separation is that we sometimes want to 'destroy' something without detaching it, if it's inside a parent element that has been destroyed.

It seems like it would make more sense to combine the two things. For one thing it results in less code. For another, I think there were cases previously where we were detaching unnecessarily; that shouldn't happen any more.

[codecov-io]

Codecov Report

Merging #1421 into master will decrease coverage by 0.15%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #1421      +/-   ##
==========================================
- Coverage   91.23%   91.07%   -0.16%     
==========================================
  Files         122      122              
  Lines        4493     4484       -9     
  Branches     1394     1400       +6     
==========================================
- Hits         4099     4084      -15     
- Misses        153      158       +5     
- Partials      241      242       +1

Impacted Files	Coverage Δ
src/compile/dom/index.ts	96% <ø> (ø)	⬆️
src/compile/nodes/Slot.ts	89.39% <100%> (-0.32%)	⬇️
src/compile/nodes/Component.ts	94.46% <100%> (-0.03%)	⬇️
src/compile/nodes/AwaitBlock.ts	96.29% <100%> (ø)	⬆️
src/compile/nodes/RawMustacheTag.ts	97.5% <100%> (+0.13%)	⬆️
src/compile/nodes/Element.ts	93.06% <100%> (ø)	⬆️
src/compile/dom/Block.ts	93.06% <66.66%> (-0.33%)	⬇️
src/compile/nodes/EachBlock.ts	97.95% <66.66%> (-0.7%)	⬇️
src/compile/nodes/IfBlock.ts	96.55% <66.66%> (-0.71%)	⬇️
src/utils/CodeBuilder.ts	80% <0%> (-5.34%)	⬇️

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