Rename the "prefix" variable to "stack"

## Feature Request

**Is your feature request related to a problem or unsupported use case? Please describe.**
In PR https://github.com/openfun/arnold/pull/427, we bikeshedded about the semantic of our "prefix" variable

**Describe the solution you'd like**
Replace "prefix" by "stack" to better describe what this variable is referring to.
