Scan: add better introspection to scan process (as in sort)

Add the following functions:

* clo_scan_get_element_size()
* clo_scan_get_sum_size()
* clo_scan_set_data()
* clo_scan_get_num_kernels()
* clo_scan_get_kernel_name()
* clo_scan_get_localmem_usage()