Optimized JavaScript property accesses when using an OCaml string as a key

The OCaml string is converted to a JavaScript string at compile time.
This make us generate better code for the functions
`Ojs.{get,set,delete}_prop_ascii` from gen_js_api.

Author: vouillon

compiler/lib/flow.ml

@@ -410,6 +410,7 @@ let the_string_of ~target info x =
 let the_native_string_of ~target info x =
   match the_const_of ~target info x with
   | Some (NativeString i) -> Some i
+  | Some (String i) -> Some (Native_string.of_bytestring i)
   | _ -> None
 
 let the_block_contents_of info x =