Convert samtools stats to htslib

Convert stats command from the legacy samtools API to the new htslib
API.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>
Add Test files for stats

These test files test some basic funtionality such as the CIGAR operator
= and sequence type =.

Signed-off-by: Martin O. Pollard <mp15@sanger.ac.uk>