[GLIMMER2] Fix ember-glimmer iterators when handling dupes in ({{#each}})

[zackthehuman]

The purpose of this PR is to fix array iteration via {{#each}} in Glimmer2 when the iteratee contains duplicate items. The following tests from ember-glimmer/tests/integration/syntax/each-test.js are now passing for Glimmer2:

• it can render duplicate primitive items
• it can render duplicate objects

Key changes:

• The order that we check objects for array-like-ness changed such that we look for Emberish arrays before plain JS arrays. Emberish arrays seemed to be falling into the plain JS array check -- which seems wrong.
• ArrayIterator and EmberArrayIterator now keep track of the keys they've seen, and when a duplicate key is seen it creates a composite key using a GUID of the array as a separator.

[chadhietala]

This should be a dict.

[chadhietala]

LGTM