Strict-mode support and unit-test fixes.

[webbju]

Checklist

• I have read the Contributor Guide
• I have read and agree to the Code of Conduct
• I have added a description of my changes and why I'd like them included in the section below

Description of Changes

• Add --strict mode which will bail out of source parsing if any issues are detected. This is the default behaviour for unit-tests.
• Fix various unit-test failures introduced whilst CI was not correctly failing on broken commits.
  • Nested struct and enum definitions are now supported.
  • Corrected "already guarded" checks in various places; supports whitespace 'ignorance' when checking present macros.

[arturjpv]

Lgtm