Tree traversals with lenses

[janmasrovira]

The following blog shows how lenses can be used as a tool for generic traversals to tree structures.

https://www.michaelpj.com/blog/2022/06/12/lenses-for-tree-traversals-redux.html

Currently the codebase of MiniJuvix has a few places that could benefit from this, mainly substitutions (in both expressions and types in MicroJuvix). It would be worth to explore if using lenses could make the code more elegant and easier to maintain.
We should post the findings (positive or negative) in this issue.

[janmasrovira]

Trying Traversals in #188