Handle slot updates where parent component has many variables

[Rich-Harris]

No related issue because the problem and solution only revealed themselves in the making of this PR.

Basically, in a situation like this...

<SomeComponent let:foo>
  {foo}
</SomeComponent>

...where the parent component has enough dynamic variables that it needs to use an array of bitmasks, the {foo} in the slot doesn't get updated. That's because the function that merges the slot scope with the ambient scope is doing bitwise operations on arrays.

This PR fixes it. I can't pretend it's the most elegant piece of code ever written, but short of a more comprehensive change to the order of operations, it's probably about as good as we can get.

[ghost]

@Rich-Harris,

Do you think this will also clear up #4077?
It looks like you made changes to the code that @mrkishi and I were looking at for that bug.

[Conduitry]

That would be a lot easier to answer if there were a reproduction for #4077.

[antony]

@Conduitry @dkondrad I've tried this branch with my repro from #4077 and sadly it doesn't fix it.

Edit: Somebody might want to check my work since I simply npm install --save-dev sveltejs/svelte#bitmask-overflow-slot to do this, and I'm not sure if there's more to it.

ant@xeno  ~/Projects/bitflow  npm run dev                    

> svelte-app@1.0.0 dev /home/ant/Projects/bitflow
> rollup -c -w

rollup v1.27.9
bundles src/main.js → public/build/bundle.js...
[!] (plugin svelte) TypeError: Cannot read property 'n' of undefined
src/App.svelte
TypeError: Cannot read property 'n' of undefined
    at Object.get expression [as expression] (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:12072:62)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:300:25)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:318:5)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:309:12)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:318:5)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:318:5)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:309:12)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:318:5)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:309:12)
    at visit (/home/ant/Projects/bitflow/node_modules/svelte/compiler.js:309:12)