The scope of the expr2ctor cache updates was incorrect.
This bug affects code of the form
```
let x := C.cases_on y ...; K[x]
```
when we try to float the `cases_on` application, and the continuation
`K[x]` contains another `cases_on` application with major `y`.
The new test exposes the bug.
This commit also fixes the case where the continuation `K[x]` projects `y`.
Fixes#26
