[C++] Added support for fixed sized list to compute functions that process lists

The following functions do not support fixed size list (and should):

• list_flatten
• list_parent_indices (one could argue this doesn't need to be supported since this should be obvious and fixed_size_list doesn't have an indices array)
• list_value_length (should be easy)

For reference, the following functions do correctly consume fixed_size_list (there may be more, this isn't an exhaustive list):

• count
• drop_null
• is_null
• is_valid