Reduce index size by using the key's array index instead of the key name

[krisk]

Description

We can reduce the index size by referencing the array index of the key, as opposed to the key text itself when constructing the $ entry of each record.

Describe the solution you'd like

Currently, the index stores keys of every entry:

[
  {
    i: 0,
    '$': {
      'title': { v: "Old Man's War", n: 0.5773502691896258 },
      'author.firstName': { v: 'John', n: 1 },
      'author.tags.name': [
        { v: 'Tag 1', i: 2, n: 0.7071067811865475 },
        { v: 'Tag 2', i: 1, n: 0.7071067811865475 },
        { v: 'Tag 3', i: 0, n: 0.7071067811865475 }
      ]
    }
  }
  /* rest of items */
]

Above, "title", "author.firstName", and "'author.tags.name" are going to be repeated for n number of entries. However, we can minimize the index size by simply using the key index:

{
  keys: [ 'title', 'author.firstName', 'author.tags.name' ],
  list: [
    {
      i: 0,
      '$': {
        '0': { v: "Old Man's War", n: 0.5773502691896258 },
        '1': { v: 'John', n: 1 },
        '2': [
          { v: 'Tag 1', i: 2, n: 0.7071067811865475 },
          { v: 'Tag 2', i: 1, n: 0.7071067811865475 },
          { v: 'Tag 3', i: 0, n: 0.7071067811865475 }
        ]
      }
    }
    /* rest of items */
  ]
}

With the above, we saved a considerable amount of space.