Make sure all tutorial code sample work with OCaml 4.14

[cuihtlauac]

OCaml version 4.14 has long term support

Therefore, tutorials should not use 5.* functions outside 5.* specific contents (effects and domains).

Run all the code samples in OCaml 4.14 and check if they work.

[0xrotense]

Hello @cuihtlauac and @SaySayo , I'm an outreachy applicant.

Please can I work on this ?

[SaySayo]

I have responded to your comment on another issue @0xrotense