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.
I just discovered that ts.forEach is actually the same as Array.prototype.some and not Array.prototype.forEach, so I'll just use that...