Implement private fields proposal

Problem

I think it would be nice to have the stage 1 private fields proposal implemented in TypeScript. It'll mostly supercede the current pseudo-private implementation, and a fully-correct implementation is only transpliable to ES6 using per-class WeakMaps. I'll note that the spec itself uses WeakMaps internally to model the private state, which may help in implementing the proposal. Currently, the proposal only includes private _properties_, but methods are likely to follow. Also, the most important part of the proposal is that private properties are only available within the class itself.