fix binding for each block local variable

[tanhauhau]

Fixes: #4757
Fixes #4517

Changes:

• remove the [bindingGroup].forEach(group => { loop to just use bindingGroup.
• when binding within a {#each}, always replace the lhs object to each_value[each_index] form. and also remove the array variable from the bind callback. ie:

<script>
    let a = [1,2,3]
</script>

{#each a as a}
    <input bind:value={a}>
{/each}

// from

let a = [1, 2, 3];
function input_input_handler(a, each_value, a_index) {
  // `a` refers to the item instead of the array
  a.foo = this.value;
  $$invalidate(0, a);
}

// to

let a = [1, 2, 3];
function input_input_handler(each_value, a_index) { // removed `a` from params when necessary
  // uses each_value[a_index] instead of `a`
  each_value[a_index].foo = this.value;

  // a refers to the array, rather than the item.
  $$invalidate(0, a);
}

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