added the incomplete string-to-bigint conversion procedure

h2/lib/h2-ascii.ads

@@ -5,6 +5,8 @@ generic
 	type Character_Type is (<>);
 package H2.Ascii is
 
+	pragma Preelaborate (Ascii);
+
 	package Pos is
 		NUL                 : constant := 0;
 		SOH                 : constant := 1;