Correctly serialize/deserialize HashReference directly to string

[augustocdias]

---

PR Submission Checklist for internal contributors

• The PR Title

  • conforms to the style of semantic commits messages¹ supported in Wire's Github Workflow²
  • contains a reference JIRA issue number like SQPIT-764
  • answers the question: If merged, this PR will: ... ³

• The PR Description

  • is free of optional paragraphs and you have filled the relevant parts to the best of your ability

---

What's new in this PR?

Implements serialization/deserialization into the HashReference directly into string, so that, maps that has it as keys can be successfully (de)serialized to json.

Issues

The MlsGroup struct cannot be serialized when there is commits pending to be merged.

Solutions

Serialize the HashReference directly into a hex string.

Testing

Test Coverage (Optional)

• I have added automated test to this contribution

References

1. https://sparkbox.com/foundry/semantic_commit_messages
2. https://github.com/wireapp/.github#usage
3. E.g. feat(conversation-list): Sort conversations by most emojis in the title #SQPIT-764.