pointerDifference() now returns UnionExp
Merge pull request #4162 from WalterBright/pointerdiff

CTFE: pointerDifference() now returns UnionExp
[Refactoring] fix code style in compilable/interpret3.d
[Refactoring] Add exceptionOrCant to make error handling shorter
Merge pull request #4163 from 9rnsr/refactor_ctfe

[Refactoring] A little code cleanup in CTFE interpreter