Values API: standardize parameters and return values

Values API: standardize parameters and return values

Changes:

* Remove `BT_VALUE_STATUS_ERROR`: the only possible error in this API is
  `BT_VALUE_STATUS_NOMEM`.

* `BT_VALUE_STATUS_CANCELED` is not considered an error: the loop did
  not finish, but it was explicitly canceled by the user.

* bt_value_copy(): return by parameter (first one, like strcpy()), and
  return status.

* bt_value_bool_get(), bt_value_integer_get(), bt_value_real_get(), and
  bt_value_string_get(): return value directly. Those functions cannot
  fail.

* bt_value_array_get_size() and bt_value_map_get_size(): return
  `uint64_t`.

* bt_value_array_is_empty() and bt_value_map_is_empty(): make them
  `static inline`.

* bt_value_map_extend(): return by parameter (like bt_value_copy()), and
  return status.

* bt_private_value_bool_set(), bt_private_value_integer_set(), and
  bt_private_value_real_set(): return `void`. Those functions cannot
  fail.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>