Optimize locally-scoped event listeners

[nolanlawson]

Currently we don't optimize event listeners that reference a local variable, for example in a for-loop:

<template for:each={list} for:item="task">
    <button key={task.id} onclick={task.delete}></button>
</template>

This produces:

on: {
    click: api_bind(task.delete),
},

Note that there is no caching on the $ctx object. Whereas there is if the referenced function is scoped to the top level (i.e. the component):

<button onclick={topScopedListener}></button>

on: {
  click: _m0 || ($ctx._m0 = api_bind($cmp.topScopedListener)),
},

Note that using this same system for locally-scoped variables is not possible, because it would lead to incorrect behavior – we don't want every task.delete in an array to be cached onto $ctx._m0, because this would mean that every event listener would just be the first task's delete.

It's possible, though, that we could do a similar caching optimization even for locally-scoped event listeners. This may be possible by using the task object itself as a WeakMap key, or by using the key as a key, or something.

[caineblood]

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