Support new matrix layout in FLINT 3.3

[fredrik-johansson]

FLINT 3.3 will change the internal representation of matrices to have an slong stride field instead of a pointer to an array of row pointers (flintlib/flint#2162).

This should not change the struct layout, but it does mean that some helpers in Nemo that access the internals need to be updated, e.g.:

mat_entry_ptr(A::ZZMatrix, i::Int, j::Int) = unsafe_load(A.rows, i) + (j-1)*sizeof(ZZRingElem)