Add libarchive helper functions for loader in unpack.c
    
    Created some helper functions for loader's interaction with
    libarchive.  There are 3 functions currently:
    
    1) A function to initialize a new archive object
    2) A function to extract all archive members to a specific
       destination (if NULL, extract to current directory)
    3) A function wrapping #1 and #2 to extract a compressed
       archive file