Implement field access and name mangling

I only just realized that fields need to be handled differently
depending on whether they're used as lvalues or rvalues. I'll handle
that when we can actually assign things to them
The same thing happens in something like
```
(new int[this.z])[0] = 15;
```
In general, the outermost construct that "uses" `onLhs` will need to reset it.