OCaml5: move @ocaml.inline annotation on the `let`

What

OCaml5 reworks the warnings for annotations: let (v [@ocaml.inline]) is no longer valid, let[@ocaml.inline] v is.

Why

Our culprit is probably ocaml/ocaml!12451

Note that this also affect protocols 16 and 17, bit it should be addressed in a follow-up MR.

How

Simply move the annotation on the let instead of the value name.

Manually testing the MR

Try to compile this file with ocaml 4 and 5:

let[@ocaml.inline always] f x y = x + y
let (g [@ocaml.inline "always"]) = fun x y -> x + y

with ocaml 4.14:

$ ocamlc test_inline.ml
# Everything works smoothly

with ocaml 5:

$ <ocaml5-opam-switch/bin/>ocamlc test_inline.ml
File "test_inline.ml", line 2, characters 9-21:
2 | let (g [@ocaml.inline "always"]) = fun x y -> x + y
             ^^^^^^^^^^^^
Warning 53 [misplaced-attribute]: the "ocaml.inline" attribute cannot appear in this context

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR