Forward comp/creds

[soerenbf]

Ref COR-1718
Closes COR-1843, COR-1850

Purpose

Forwards compatibility for anything credential-related. This also introduces a type Known<T> which unwraps Upward recursively. Should be used with care though, as type information is lost when stripping, as it does not work with classes (which we use to strongly type our domain types).

Changes

• Wraps VerifyKey references in Upward

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.