Wishlist: improve list element access

[gbonnefille]

In a mal_long_list, element management requires to retrieve and update two raw / internal arrays:

• the array containing data
• the array containing presence flags

Of course, having to use two arrays for a single need (adding or retrieving one data element) is necessary source of error.

It would be simpler (and more robust) for the user to have functions wrapping these actions. For example:

• void mal_long_list_get_element(mal_long_list*, long*, bool*); or bool mal_long_list_get_element(mal_long_list*, long*);
• void mal_long_list_add_element(mal_long_list*, long, bool);

Of course, this change is required for all other lists.