Allow multiEntry + Array keyPath

[inexorabletash]

Imported from https://www.w3.org/Bugs/Public/show_bug.cgi?id=21836

Currently, the spec for IDBObjectStore.createIndex has:

If keyPath is a sequence and multiEntry is set, throw an InvalidAccessError exception.

This disallows some interesting use cases, although the expected behavior in more complex cases is unclear. Per @sicking:

Consider a multiEntry index with keyPath ["a", "b"]

It's pretty intuitive that storing a value like:

{
  id: 1,
  a: "apple",
  b: "orange"
}

would add the following entries to the index:

"apple" -> 1
"orange" -> 1

But what would storing the following value result in?

{
  id: 1,
  a: ["apple", "x"],
  b: ["orange", "y"]
}

Would this result in the following entries

A)
["apple", "x"] -> 1
["orange", "y"] -> 1

or

B)
"apple" -> 1
"x" -> 1
"orange" -> 1
"y" -> 1

or

C)
["apple", "orange"] -> 1
["x", "y"] -> 1

[inexorabletash]

If we ever tackle this, we also need to specify the behavior when a subkey is missing, e.g.:

store.createIndex('by_name', ['name', 'alias'], {multiEntry: true});
store.add({name: 'Luke Cage', alias: 'Power Man'}); // OK; two entries
store.add({name: 'Jessica Jones'}); // ???

If multiEntry were false, this throws since alias does not resolve and the compound key can't have a hole. But here it would be logical (?) for there to be only a single entry.

[theSherwood]

@inexorabletash
This would be very useful. I currently have an app in dire need of it. Though, wouldn't this index make the most sense?:

D)
["apple", "orange"] -> 1
["apple", "y"] -> 1
["x", "orange"] -> 1
["x", "y"] -> 1

Of course, this risks a combinatorial explosion. But it seems to me that the combination of multiEntry and compound indices ought to be the cartesian product. Thoughts?