Generate version.ml from satysfi.opam to make releasing easy

[y-yu]

• When releasing (for example Release SATySFi 0.0.10 #418), the releaser(= gfn-san) rewrite the version of src/frontend/main.ml by their hands
• tools/genversion will generate such a version.ml 👇 when building

  let version = "0.0.9"

  and genversion gets version string ("0.0.9") from satisfy.opam using my ad-hoc regex
• Then src/frontend/main.ml uses the generated version string
• I think that this PR reduces a step of releasing process 🤔

[na4zagin3]

How about having OPAM run substitute commands like na4zagin3/satyrographos#189 (dune subst and sed)?

[y-yu]

Thanks for the comment 😄 I saw the technique of na4zagin3/satyrographos#189.
First of all, the best choice is what gfn-san likes, I think. Based on that, my opnions about this PR (using OCaml way) v.s. dune and sed(zagin-san's way) are:

• zagin-san's way is simpler than my PR way
• In sed, there are some differences between BSD and GNU so I don't like to use sed😇
  • Indeed, on this usecase, sed does simple replacement only so is it no matter? 🤔
  • OCaml is more complecate than sed but it's the same behavior on BSD and GNU
• OS commands and shell scripts are not unit testable

[y-yu]

Even if we'll choice either way, it would be better to write the version info to the other file (such like version.opam) rather than using single OPAM file 🤔 in order to manipulate it by GitHub Actions(automatic release scripts) or so on. (but it's not possible on the current OPAM)
It may be a lesson for SATySFi 0.1.0 package manager setting file 👀

[gfngfn]

I'm so sorry for the late response.

I can't confidently tell which of the two approaches is better, but at least I like Yoshimura-san's OS-independent implementation. I would thus like to merge this PR for the moment.

Thank you for the improvement!

[na4zagin3]

I'm so sorry for my long silence. Avoiding using sed achieves OS-independency obviously, which is nice! This is too late but anyway I retract my comment. Thank you!