core: drop line iterators

Hand-crafted iterators don't make much sense in C and impose an
additional layer of memory allocation and resource releasing. Remove
the line iterators from the core C library.

We're leaving the iterators where they make sense: in C++ and Python
bindings but we convert them to using other means of keeping track of
lines.

Signed-off-by: Bartosz Golaszewski <bgolaszewski@baylibre.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>