[Refactoring] Tweak code style in interpreter
[Refactoring] A small code flow improvement in AndAndExp and OrOrExp
[Refactoring] clean up statement interpretation
Simplify CallExp::interpret() and support DotTypeExp in CTFE

Handle function values in each CTFE steps, then interpreting CallExp::e1 just only once will be enough.

Problematic case is a __cpctor call. In it, the 'this' expression is an
unilitialized variable so we need to initalize it before the interpretation of e1.

Also simplify ThisExp::interpret, comes from CallExp improvement.
Stop generating internal __cpctor function so it's just redundant

Instead we can directly memcpy the object, then call the postblit.
Simplify CTFE interpreter

- Merely distinguish CTFE values and CTFE references.
Add CtfeResult