IDB: use a multiEntry index and 'structured' keys

[leplatrem]

As suggested by Andrew in https://bugzilla.mozilla.org/show_bug.cgi?id=1486980#c36

Example pseudocode:

objectStore.createIndex("app_indices", "appIndices", { multiEntry: true });

function fancyPut(idbDataStore, collectionName, collectionIndicesMap, appObj) {
  const appIndices = collectionIndicies.map([indexName, indexEvalPath]) => {
    // for the example, assume the key is not nested...
    const appVal = appObj[indexEvalPath];
    if (Array.isArray(appVal)) {
      throw new Error("The value can't be an array for our current bounds cleverness.");
    }
    // Create an index key that's array-structured so we can bound its
    // key-space.
    return [collectionName, indexName, appVal];
  });
  const wrappedObj = {
    actualObj: appObj
    appIndices
  };
  return idbDataStore.put(wrappedObj);
}

async function findWithIndex(idbDataStore, collectionName, indexName, appExact,
[appLow, appHigh]) {
  // [collectionName, indexName] lower-bounds all [collectionName, indexName, ...]
  // because a shorter array is by definition less than a longer array that is
  // equal up to their shared length.
  // [collectionName, indexName, []] upper-bounds all [collectionName, indexName, ...]
  // because arrays are always greater than strings/dates/numbers.  So as long
  // as the other contents of the index are non-arrays, we're good.
  const wholeIndexBounds = IDBKeyRange.bound(
    [collectionName, indexName],
    [collectionName, indexName, []],
    true, true); // exclusive range for the edge-case values.
  const exactIndexBounds = IDBKeyRange.only([collectionName, indexName, appValue]);
  const rangeBounds = IDBKeyRange.bound(
    [collectionName, indexName, appLow],
    [collectionName, indexName, appHigh],
    false, false); // assume caller meant inclusive.
  // DO ACTUAL INDEX STUFF
  return wrappedObj.actualObj;
}