Sharing

`let x = pi in x + x` doesn't reuse work in the shared computation of `pi`. It reduces to `pi + pi`, which is
```
proc () -> do
  x1 <- pi -< ()
  x2 <- pi -< ()
  (+) -< (x1, x2)
```

whereas we would prefer
```
proc () -> do
  x <- pi -< ()
  (+) -< (x, x)
```
I'm not sure how we can do that.