Use efficient String.starts_with implementation

The existing implementation for `Compat.String.starts_with` needlessly
makes a copy of the prefix. This patch instead uses the implementation
of `String.starts_with` from recent versions of the OCaml standard
library, which avoids the copy.

src/lib/util/compat.ml

@@ -1,11 +1,22 @@
+(* Locally restore the default polymorphic operators of the Stdlib, to make it
+   easier to copy code from there without modifications. *)
+open Stdlib
+
 module String = struct
   open Stdlib.String
 
   let starts_with ~prefix s =
-    length s >= length prefix &&
-    equal (sub s 0 (length prefix)) prefix
+    (* Copied from stdlib/string.ml in OCaml 5.0 *)
+    let len_s = length s
+    and len_pre = length prefix in
+    let rec aux i =
+      if i = len_pre then true
+      else if unsafe_get s i <> unsafe_get prefix i then false
+      else aux (i + 1)
+    in len_s >= len_pre && aux 0
 
   let fold_left f x a =
+    (* Copied from stdlib/bytes.ml in OCaml 5.0 *)
     let r = ref x in
     for i = 0 to length a - 1 do
       r := f !r (unsafe_get a i)