Preserve as much as possible the initial OCaml variable names

[hhugo]

• move closure compilation pass from JavaScript to Intermediate ast
• Improve tail call optimization pass - do not generate useless closures.
• Preserve as much as possible the initial OCaml variable names

[hhugo]

@vouillon, this PR does not include the optimization for multiple closure inside a loop [1]. It seems that this optimization is never used when building examples & toplevel. I'm considering not adding the optimization back. What do you think ?

[1] https://github.com/ocsigen/js_of_ocaml/blob/master/compiler/generate.ml#L1243

OCaml code

  let l = ref [] in
  for i = 0 to 2 do
    let rec f1 x = if x = 0 then i else f2 (x -1)
    and f2 x  = if x = 0 then i else f1 (x - 1)
    in l:= f1 :: f2 :: !l
  done;

origin/master

var
       funenv_FX_=
        function(i_FS_)
          {function f1_FV_(x_FT_){return 0===x_FT_?i_FS_:f2_FU_(x_FT_-1|0)}
           function f2_FU_(x_FW_){return 0===x_FW_?i_FS_:f1_FV_(x_FW_-1|0)}
           return [f1_FV_,f2_FU_]}
         (i_FS_),
       f1_FV_=funenv_FX_[0],
       f2_FU_=funenv_FX_[1];

This PR

var
       f1_FV_ =
        function(i_FS_)
          {return function f1_FV_(x_FT_){return 0===x_FT_?i_FS_:f2_FU_(x_FT_-1|0)}}(i_FS_)
      f2_FU_  =
        function(i_FS_)
          {return function f2_FU_(x_FW_){return 0===x_FW_?i_FS_:f1_FV_(x_FW_-1|0)}}(i_FS_)

[vouillon]

I think it's ok to remove this optimization.