After this commit, we have to use an explicit `discard` in code such as
```
def g (x : Nat) : IO Nat := ...
def f (x : Nat) : IO Unit := do
discard <| g x -- type error without the `discard`
IO.println x
```
Motivation: prevent users from making mistakes such as
```
def f (xs : Array Nat) : IO Unit := do
xs.set! 0 1
IO.println xs
```
when they meant to write
```
def f (xs : Array Nat) : IO Unit := do
let xs := xs.set! 0 1
IO.println xs
```
@kha I figured out why we had a long pause in the end of this benchmark
when using `11` instead of `9`. The function `deriv` was computing
`d := d "x" f` (the expensive computation), printing the size of `f` and
returning `d`. So, in the last step we were quickly printing the size
of the input 40230090 (when using `nest deriv 11 f`), and then computing
`d := d "x" f` which returns an object of size 374429936 which is never
used for anything.
That is, the pause had nothing to do with memory deallocation. I found
this issue after I implemented the deferred free feature which did not
fix the pause :)
