Explicitly call out dev-dependencies used in solutions

[mgeisler]

From #1288 (comment): the safe FFI wrapper solution does not actually work when you copy-paste it into a new Cargo project. The problem is that the unit tests rely on tempfile.

We need to fix this: I suggest adding some text to the solution which shows the cargo add commands to run. Alternatively, we could {{#include ...}} the Cargo.toml file — but I fear this will become messy since all solutions currently share a single Cargo.toml file. We could perhaps split them up further?