Move Struct/ClassDeclaration::toDt into free functions
Move ClassReferenceExp::toDt* into free functions
Merge pull request #3318 from yebblies/classtodt

[DDMD] [refactor] Move remaining toDt methods into free functions
[cleanup] Tweak style of runnable/mangle.d
[cleanup] Gather `mangleof` property related tests into runnable/mangle.d
[cleanup] Adjust test results in runnable/mangle.d