[Refactoring] Integrate code for making nested aggregate

- Move StructDeclaration::makeNested to AggregateDeclaration
- Change AggregateDeclaration::isnested to Dsymbol*
add apply() for statements
Merge pull request #1814 from WalterBright/addsapply

add apply() for statements
Merge pull request #1813 from 9rnsr/refactor_nested

[Refactoring] Integrate code for making nested aggregate