Implement private variables, getter/setters and methods using Symbols

Example implementation for #684 

Not for merging, only discussion.

TODO:
- Implement compiler flag.
- Use compiler flag to selectively turn on new codegen.
- Only generate __symbol declaration when the current sourceFile has any classes with private members. The currnet code always generates it.
- Tests.
- ~~Handle static and instance members with the same name.~~ Done.
The rest of the file uses double-quotes and escape double-quotes inside, so I'll stick to that for consistency.