Add String.is_empty

src/core/CCString.cppo.ml

@@ -52,6 +52,8 @@ let init = String.init
 
 let length = String.length
 
+let is_empty s = equal s ""
+
 let rev s =
   let n = length s in
   init n (fun i -> s.[n-i-1])

src/core/CCString.mli

@@ -54,6 +54,9 @@ val equal : string -> string -> bool
 
 val compare : string -> string -> int
 
+val is_empty : string -> bool
+(** @since NEXT_RELEASE *)
+
 val hash : string -> int
 
 val init : int -> (int -> char) -> string