changeset.snapshot() does not work with nested keys

[SergeAstapov]

Here is quick reproduction: https://codesandbox.io/s/infallible-meadow-yzt61

Looks like it's been working just fine in ember-changeset@2 because this._changes had format:

{
  "firstName": "Jim",
  "lastName": "B",
  "address.city": "NYC",
  "address.state": "NY"
}

validated-changeset has structure like

{
  "firstName": "Jim",
  "lastName": "B",
  "address": {
    "city": "NYC",
    "state": "NY"
  }
}

so this iteration fails https://github.com/validated-changeset/validated-changeset/blob/master/src/index.ts#L558-L561

Also, changes getter does not work as expected as it returns

[
  {
    "key": "firstName",
    "value": "Jim"
  },
  {
    "key": "lastName",
    "value": "B"
  },
  {
    "key": "address.city",
    "value": "NYC"
  },
  {
    "key": "state",
    "value": "NY"
  }
]

[SergeAstapov]

This should be reopened as #25 fixed only part of the issue.

@snewcomer I tried to look into what's up with snapshot()/restore() - looks like it has the same issue as get changes had and does not play nice with nested keys. I'll submit PR to fix it in the same way as I did in #25.