Change representation of byte sequences from String to Bytes resulting in representation as Data in swift

[soerenbf]

Purpose

Representation of data as hex strings is good for readability only, which should not be the core of our data structures. This PR changes the representation of raw data to Bytes and consequently Data in the generated swift code in any case where hex strings were previously used.

Checklist

• My code follows the style of this project.
• The code compiles without warnings.
• I have performed a self-review of the changes.
• I have documented my code, in particular the intent of the
  hard-to-understand areas.
• (If necessary) I have updated the CHANGELOG.