Avoid polymorphic comparison over float values

[xclerc]

While randomly walking through the code, I noticed something similar
to an old flambda bug. Indeed, with master, the following code (copied
from this pull request) prints inf instead of -inf:

let z =
  let x = -0. and y = +0. in
  if mod_float x 1. >= 0. then
    x
  else if false then x else y

let () =
  Printf.printf "%g\n" (1. /. z)

The culprit is the use of the polymorphic equality on float values by a join
operation in the Flow module. This pull request ensure that the equality
function over Code.constant value does not use the polymorphic equality.

[hhugo]

Can you add a changelog entry ?

[xclerc]

I have added a changelog entry and pushed additional commits
to support OCaml 4.0{2,3,4}.

[hhugo]

I've reformated the code with make fmt (depends on ocamlformat)
and moved foat_equal, int64_equal, array_equal in stdlib.ml

[hhugo]

Thanks for the bugfix