Use Object.create(null) instead of {} to compute keyed diffs

[jfmengels]

This is a minor improvement pull request.

{} creates an empty object but which still contains methods and properties like toString. Checking changes[key] when the key is for instance toString creates false positives in that check.

I was looking whether into whether this caused bugs, but in practice, the algorithms in _VirtualDom_insertNode and _VirtualDom_removeNode end up with the correct behavior because they recurse with a modified key, but do so with an unnecessary iteration.

Object.create(null) creates an empty object without any methods or properties, leading to no false positives and one less unnecessary iteration.

[mozfreddyb]

Why not Map()?

[jfmengels]

Because Elm currently compiles down to ES5, where Map is not available.