IH-553: Optimize Sexpr.escape

It was allocating a string for each character, only to then immediately add it to a buffer.
Instead add the character to the buffer directly.

The temporary string was probably created to factor out 'Buffer.add',
but I'd rather copy Buffer.add 4x times than have the perf hit.

Further optimizations might be possible here as the comments says,
but this is a low-risk, obvious optimization.

Signed-off-by: Edwin Török <edwin.torok@cloud.com>

ocaml/libs/sexpr/sExpr.ml

@@ -39,18 +39,15 @@ let escape s =
   let escaped = Buffer.create (String.length s + 10) in
   String.iter
     (fun c ->
-      let c' =
-        match c with
-        | '\\' ->
-            "\\\\"
-        | '"' ->
-            "\\\""
-        | '\'' ->
-            "\\\'"
-        | _ ->
-            Astring.String.of_char c
-      in
-      Buffer.add_string escaped c'
+      match c with
+      | '\\' ->
+          Buffer.add_string escaped "\\\\"
+      | '"' ->
+          Buffer.add_string escaped "\\\""
+      | '\'' ->
+          Buffer.add_string escaped "\\\'"
+      | _ ->
+          Buffer.add_char escaped c
     )
     s ;
   Buffer.contents escaped