Make storing IDs in arrays consistent with the rest of the store

[stubailo]

I was hacking around with some ideas for inspecting the store, and ran into a problem - ID references are not always stored the same way.

When it's an object field, the reference looks like:

fieldKey: {
  type: 'id',
  id: 'asdf',
  generated: false,
}

When it's an array field, it's not wrapped at all:

fieldKey: [
  'asdf',
  'qwer',
]

This is problematic if one wants to write a tool that can follow query trees through the store. Looks like when we initially refactored to escape these IDs in the store, we didn't go all the way and totally forgot about arrays.

This PR fixes it to look like:

fieldKey: [
  { type: 'id', id: 'asdf', generated: false },
  { type: 'id', id: 'qwer', generated: false },
}

Note: This involved updating some of the old mutation behavior code. We should deprecate that and delete it/move it to an auxiliary package.

Questions:

• Is this a breaking change? I suppose in some ways it is, because it changes the store format. But I would be surprised if people were relying on the old behavior because someone else would have brought up that it was inconsistent?

TODO:

• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

[helfer]

@stubailo lgtm, shall I merge this before the next release?

[stubailo]

Yes, can you update the CHANGELOG as well? I forgot to do it and I'm not near a terminal at the moment.