Deprecate/remove OCaml stdlib functions

[cknitt]

As discussed with @zth: We should deprecate most functions from the OCaml stdlib and point people to Belt / Js.xxx2 / a new stdlib instead where appropriate.

So basically one would have to go through all files in jscomp/stdlib-406 and decide which functions are still needed. E.g. parts of Pervasives like string_of_int, print_endline, ... can be deprecated, as well as probably all functions in Array, List, Set etc. (although Array.get is a bit of a special case).

It may be tricky though where exactly to point the user in the deprecation messsage as long as there is no new stdlib in place yet.

[cknitt]

I think this is something for later on when Core is moved into the compiler. Moving to v12.

[cknitt]

Or already fully remove the OCaml stdlib + related primitives in v12, which would make revamping the stdlibs build procedure (#6826) much easier.

[cknitt]

Related: We will still need some sort of Pervasives and need to decide what should stay in there.