Simpler & less redundant IFormattable implementation

Instead of hopping over another String.Format (and causing an allocation of another format string), delegate to the underlying implementation of IFormattable on the value if it exists, otherwise to Option's own ToString().

Optional/Option_Maybe.cs

@@ -392,11 +392,9 @@ public Option<T> Filter(Func<T, bool> predicate)
         /// <returns>The filtered optional.</returns>
         public Option<T> NotNull() => hasValue && value == null ? Option.None<T>() : this;
 
-        string IFormattable.ToString(string format, IFormatProvider formatProvider)
-        {
-            return hasValue
-                 ? string.Format(formatProvider, "{0:" + format + "}", value)
-                 : string.Empty;
-        }
-}
+        string IFormattable.ToString(string format, IFormatProvider formatProvider) =>
+            !hasValue
+            ? string.Empty
+            : (value as IFormattable)?.ToString(format, formatProvider) ?? ToString();
+    }
 }