We need a PPX lens generator

It would save a lot of boilerplate code – and make lenses a ridiculously useful feature – to have a PPX extension allowing to automatic lenses generation, so that we could write

``` ocaml
type car = {
    make : string;
    model: string;
    mileage: int;
  } [@@lens]
```

and have automatically `make_lens` etc. defined for us.
