NFData instance for DecoderError

[dancewithheart]

Fixes #643

Description

• add NFData instance for DecoderError
• use it in assertIsLeft to force decode into normal form.

Tested with:

cabal build cardano-binary
cabal test cardano-binary

Checklist

• Commit sequence broadly makes sense and commits have useful messages
• New tests are added if needed and existing tests are updated
• All visible changes are prepended to the latest section of a CHANGELOG.md for the affected packages.
  New section is never added with the code changes. (See RELEASING.md)
• When applicable, versions are updated in .cabal and CHANGELOG.md files according to the
  versioning process.
• The version bounds in .cabal files for all affected packages are updated.
  If you change the bounds in a cabal file, that package itself must have a version increase. (See RELEASING.md)
• Self-reviewed the diff