User land polyfill c stub in OCaml syntax

[bobzhang]

The current way of polyfill C stubs in js syntax is as below

require('bs-platform/lib/js/caml_external_polyfill.js').register('caml_fancy_add',function(x,y){
  return + ((""+x ) + (""+y))
})

we may allow user to polyfill it in OCaml as well, suggested syntax is

[%register "caml_fancy_add", fun x y -> x + y ]

[stale]

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