Add new API function ihex_get_address_range()

This new function allows to determine the actual data range contained in an ihex_record_set_t. It iterates through the records much like ihex_mem_copy() and tracks the minimum and maximum content addresses.

The API documentation contains an example of how to get the needed data range size for copying.

As usual, function name is up for discussion. No tests yet, should probably be added before merging to master.