Add roundtrip test functions skipping comparison with FlatTerm-decoded

[teodanciu]

Description

This is an attempt to allow for more relaxed cbor roundtrip checks, namely: optionally skipping the comparison of the initial value with the value obtained by decoding the FlatTerm. For types that are memoized, this check doesn't succeed (and it's also already skipped roundtrip functions for Annotator types) .
Since we'll have DecCBOR instances for types for which atm we only have DecCBOR (Annotator) instances (memoized), if we can't skip this check, then we can't have roundtrip checks for them.

PR is a draft, and missing comments and changelog entries, because it's just a proposal - I'm not too happy about polluting the Roundtrip space with another dimension.
I would be happy to implement a more elegant solution, or maybe better names.

Checklist

• Commits in meaningful sequence and with useful messages
• Tests added or updated when needed
• CHANGELOG.md files updated for packages with externally visible changes
  
  New section is never added with the code changes. (See RELEASING.md)
• Versions updated in .cabal and CHANGELOG.md files when necessary, according to the
  versioning process.
• Version bounds in .cabal files updated when necessary
  
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Code formatted (use scripts/fourmolize.sh)
• Cabal files formatted (use scripts/cabal-format.sh)
• hie.yaml updated (use scripts/gen-hie.sh)
• Self-reviewed the diff