Add a Compare module to the standard library

[kit-ty-kate]

First proposed in #13753 (comment)

As previously extensively discussed in #9928, #9080 and in many tickets or forum posts, both the physical equality operator == and the polymorphic comparison functions compare and = are known footguns for OCaml developers of all levels.

Similarly to #9080 / #13753, this PR hopes to be a first step towards a world where OCaml does not have this footgun anymore by encouraging people to use functions from this new module (which is more explicit than using it directly from Stdlib and thus less prone to footgunning), then deprecating ==, compare, … when they are less used.

Note for reviewers: i'll add documentation to the functions once/if the interface is agreed

[nojb]

I am in two minds about these submodules.

The main use of them is with numeric types (Int, Int64, Char, maybe String, etc). If anything, monomorphic versions of these operators should be added to those modules. I don't think the operators <, >, <=, >= are very useful in contexts where one is explicitly using a polymorphic compare (since the ordering is implementation-defined). As for = and <>, they are useful, but once you have put them inside a submodule, I don't see myself using a local open to save a few character with respect to Compare.Poly.equal.

In other words, perhaps it would be simpler not to include the Syntax submodules at all (which would also mean that we do not have to discuss what to name the submodule).

[gasche]

In other words, perhaps it would be simpler not to include the Syntax submodules at all (which would also mean that we do not have to discuss what to name the submodule).

Yes, that would also be my preference to reduce decision fatigue.