[Refactoring] Add StructDeclaration::fit to reuse it in other places
Supplement fix for DIP29 feature: Do not emit CastExp for const and unique conversions.
fix fix Issue 12681 - Rewrite rule prevents unique detection

Do rewriting in glue-layer.
fix CTFE interpreter
Merge pull request #3522 from 9rnsr/fix12681

Issue 12681 - Rewrite rule prevents unique detection