/*
 * call-seq:
 *   compress(str)
 *
 * Shortcut for compressing just a string.
 *
 *    Bzip2.uncompress Bzip2.compress('data') # => 'data'
 *
 * @param [String] str the string to compress
 * @return [String] +str+ compressed with bz2
 */
static VALUE bz_compress(int argc, VALUE *argv, VALUE obj) {