Add decode_opt, an option-returning variant of decode

src/b64.ml

@@ -59,6 +59,12 @@ let decode ?alphabet input =
   done;
   Bytes.unsafe_to_string output
 
+let decode_opt ?alphabet input =
+  try
+    Some (decode ?alphabet input)
+  with
+    Not_found -> None
+
 let encode ?(pad=true) ?alphabet input =
   let length = String.length input in
   let words = (length + 2) / 3 in (* rounded up *)

src/b64.mli

@@ -39,6 +39,9 @@ val uri_safe_alphabet : string
     @raise Not_found if [s] is not a valid Base64 string.  *)
 val decode : ?alphabet:string -> string -> string
 
+(** Same as [decode], but returns [None] instead of raising. *)
+val decode_opt : ?alphabet:string -> string -> string option
+
 (** [encode s] encodes the string [s] into base64. If [pad] is false,
     no trailing padding is added.
     [pad] defaults to [true], and [alphabet] to {!default_alphabet}. *)