This PR adds the benchmark vcgen_reader_state that is a variant of
vcgen_add_sub_cancel that takes the value to subtract from a `ReaderT`
layer. Measurements:
```
goal_100: 201 ms, 1 VCs by sorry: 0 ms, kernel: 52 ms
goal_500: 382 ms, 1 VCs by sorry: 0 ms, kernel: 327 ms
goal_1000: 674 ms, 1 VCs by sorry: 1 ms, kernel: 741 ms
```
Which suggests it scales linearly. The generated VC triggers superlinear
behavior in `grind`, though, hence it is discharged by `sorry`.
bb8d8da1af