fix: change index key visibility

Change the visibility of the key enum to
pub(crate) so the database can be used inside the crate.

The enum can then be reexported if required in another module.

struct_db_macro/src/struct_db.rs

@@ -121,7 +121,8 @@ pub fn struct_db(args: TokenStream, input: TokenStream) -> TokenStream {
             }
         }
 
-        enum #keys_enum_name_token {
+        /// Index selection Enum for [#struct_name]
+        pub(crate) enum #keys_enum_name_token {
             #(#keys_enum_tokens),*
         }