[m-rev.] for review: add examples of the C data passing conventions

[fabrice nicol]

Hi Julien,

this is a welcomed addition that will save time and pain to many 
beginners and (somewhat) more advanced testers alike.

I would perhaps add some lines of code giving an example of how tuples 
and array2d's are passed from Mercury to C and back.

Related to this, I would give an example of how data tables of any sort 
can be passed to C as arrays of user-defined types (see this thread: 
http://lists.mercurylang.org/archives/users/2021-May/009119.html). It is 
not an obvious issue, though one of those that users may want to handle 
without too much pondering.

Fabrice