Implement make_ex in CakeML

Following #350 we now have a small SML program to generate the tutorial exercises, currently at `tutorial/solutions/make_ex.sml`. This issue is to re-implement this program in CakeML instead (including automatically generating the executable and using it in the relevant `Holmakefile`), in part to ensure our infrastructure is up to scratch.

The program might also be better in the `developers` directory, but that will depend on how the "build a CakeML program" aspect is handled.

Start by reading tutorial/solutions/make_ex.sml and the relevant Holmakefile, then review issue #350 for the existing exercise-generation context. Determine how the CakeML program should be built and whether it belongs in tutorial or developers. Done means the generator is reimplemented in CakeML, its executable is generated automatically, and the relevant Holmakefile uses it.

Written by the indexing model from the issue text.