Calculate XPT row_count using metadata and final chunk

[gerrycampion]

Describe the issue

For XPT files, I understand that row_count cannot be extracted from the metadata alone, but I think it can be calculated using only the metadata and final 80-byte chunk.

Expected behavior

• Read the header information to find: variable_sizes and the start of record data
• Calculate record_size as sum of variable_sizes
• Read the last 80-byte chunk of data to find out how much trailing ASCII blank padding there is.
• Calculate number of records using:
  (total_file_size - start - padding) / record_size

It would be helpful if readstat could expose either:

• row_count

or these, if not already exposed:

• total_file_size
• records_start_offset
• records_end_offset