I've added comments in the code to document these functions.  I have no
opinion where they live except they should probably be available to
extensions modules. 

These routines increment an N-length counter representing a position in
an N-dimensional array with wrap-around when the counter reaches the
size of the dimension.  Thus, for a (2,3) array we have:

F-version

0,0
1,0
2,0
0,1
1,1
2,1

C-version
0,0
0,1
0,2
1,0
1,1
1,2