flambda-backend: inline fast path of caml_applyN (#934)

* Add compilation flag -caml-apply-inline-fast-path
* Use -caml-apply-inline-fast-path for main build and libraries
* Inline fast path of caml_apply
* Bind args
* Refactor and apply the same transformation to Flamba 2.
* Pass [ty] argument to [call_caml_apply]
* Add a CR about a version of caml_applyN that has only cold path
and update a comment.

dune

@@ -25,7 +25,7 @@
    ;; CR gyorsh: it is not clear what the ":standard" flags are, and they
    ;; may change depending on the version of dune.
    ;; Consider hard-coded flags, such as -O3.
-   (:standard -alloc-check)))
+   (:standard -alloc-check -caml-apply-inline-fast-path)))
  (boot
   (flags
    (:standard -warn-error +A))))