Avoid generating calls to Curry when adjusting arity of uncurried functions.

A function such as this one:
```
let t4 = ({contents: x0}, {contents: x1}, {contents: x2}, {contents: x3}) => (x0, x1, x2, x3)
```

is 4 nested functions each one extracting the `contents` field.
In uncurried mode, the arity is adjusted to be 4, by putting a wrapper on top.
The wrapper was added by applying all the new args at once, which requires `Curry` runtime.

Author: cristianoc

CHANGELOG.md

@@ -58,6 +58,7 @@
 - Make tests ready for uncurried mode. https://github.com/rescript-lang/rescript-compiler/pull/6862
 - Make gentype tests uncurried. https://github.com/rescript-lang/rescript-compiler/pull/6866
 - Ignore `@uncurry` attribute in uncurried mode, to avoid generating calls to `Curry` at runtime. https://github.com/rescript-lang/rescript-compiler/pull/6869
+- Avoid generating calls to Curry when adjusting arity of uncurried functions. https://github.com/rescript-lang/rescript-compiler/pull/6870
 
 #### :nail_care: Polish
 

jscomp/core/lam_eta_conversion.ml

@@ -163,9 +163,13 @@ let unsafe_adjust_to_arity loc ~(to_ : int) ?(from : int option) (fn : Lam.t) :
             let extra_args =
               Ext_list.init (to_ - from) (fun _ -> Ident.create Literals.param)
             in
+            let rec mk_apply body vars = match vars with
+              | [] -> body
+              | var :: vars ->
+                mk_apply (Lam.apply body [var] ap_info) vars in
             Lam.function_ ~attr:Lambda.default_function_attribute ~arity:to_
               ~params:(Ext_list.append params extra_args)
-              ~body:(Lam.apply body (Ext_list.map extra_args Lam.var) ap_info)
+              ~body:(mk_apply body (Ext_list.map extra_args Lam.var))
         | _ -> (
             let arity = to_ in
             let extra_args =