Include public domain implementation of md5 by Colin Plumb in 1993 where no
copyright is claimed.

Source: http://home.educities.edu.tw/fbtip/dl_software.html#otherwise
add extern "C" for cpp
Include public domain implementation of crc32 from http://www.csbruce.com/software/crc32.c
expose Crc32_ComputeBuf from crc32.c
Implement hash_without_intercept which hash the column names of model matrix except "(Intercept)"
debug: typo of __cplusplus
debug: segment fault
debug: naming collision with crc32
remove debug message
append mapping
add unit test to check matrix-vector multiplication