Small changes required for the upcoming "precompiled" version of learn-ocaml

[AltGr]

Some quirks needed in porting to the pre-compiling version of learn-ocaml:

• remove #install_printer (and any other) directives: we are no longer in a toplevel so these are no longer available. However, using tricky hacks, you can still define a printer for type foo with just let print_<foo> ppf x = ... Format.fprintf ppf ...

• replacing open intended for shadowing with include in Prepare/Prelude
  (fpottier/unionfind/prepare.ml, mooc/week4/seq3/ex1/prepare.ml)
  Open can't reach outside of its compilation unit

• renamed multiple definitions of exceptions/types/modules with the same name
  (fpottier/tictactoe/test.ml)
  this is allowed in the toplevel but not by the OCaml compiler

• renamed toplevel functions named sample_* that don't type as samplers
  (mooc/week3/seq4/ex1/test.ml, mooc/week5/seq1/ex1/test.ml)
  These are dynamically typed later on, which just issues a warning, so we could
  disable this less precise check altogether for better compat, but it seems
  cleaner to reject them

• avoid using a default sampler then overriding it
  (mooc/week5/seq2/ex1/test.ml)
  Due to difficulties in name resolution, this is forbidden, but the workaround
  is simple: just alias the built-in sampler locally before the first use.

[erikmd]

Thanks @AltGr!

Question: regarding your comment

remove #install_printer (and any other) directives: we are no longer in a toplevel so these are no longer available. However, using tricky hacks, you can still define a printer for type foo with just let print_<foo> ppf x = ... Format.fprintf ppf ...

could you be more specific? I mean, from you PoV is there something to do in this PR (with the hack you suggest or so) before it is merged?

Otherwise, I propose to merge it as is now, as it is backward-compatible (cf. CI ✔️)

[AltGr]

could you be more specific? I mean, from you PoV is there something to do in this PR (with the hack you suggest or so) before it is merged?

EDIT: to be clear, the tricky hacks are hidden in the implementation of learn-ocaml, they're not in the exercises!

Normally it should be fine, what I meant is that registration of the printers now only relies on proper naming print_foo with the defined type foo in scope, and proper typing as Format.formatter -> foo -> unit (with extra arguments in front for the printing of the type parameters in the case of a parametric type, e.g. 'a foo, ('a, 'b) foo, etc.)

From my tests, the existing exercises don't leverage parametric type printing (it's a feature of OCaml with #install_printer already, but undocumented 🤷 ) — although the markov exercise typically would benefit from it ; and already have aptly named printers so they should work as expected with this PR.