For example, in the new test `qsort64.lean`, the new optimization
prevents the repeated execution of `box UInt64.inhabited`.
On my machine
```
./run.sh qsort64.lean 2000000
```
Goes from 1.22s to 0.355s
Remark: the attribute actions used by the frontend are all in IO.
These actions access attributes by name, and need access to the IO.ref
that contains all registered attributes in the system.