encapsulate memcpy with constructor
Merge pull request #4169 from WalterBright/memcpyctor

CTFE: encapsulate memcpy with constructor
[Refactoring] Add findGotoTarget to share common code
[Refactoring] Return NULL from WithStatement::interpret()
[Refactoring] Remove Statement::interpret()
[Refactoring] Remove Expression::interpret()
[Refactoring] Handle NULL Expression or Statement in interpret() call