use local _argn instead of ARGN for efficiency

It seems in a macro doing something like
  message("${ARGN}")
is ***much*** slower than
  set(_argn ${ARGN})
  message("${_argn}").
I don't know why, but this fixes it in the
pack/unpack macros that get called a lot.
Merge pull request #1 from hiDOF-forks/feature/argn_speedup

use local _argn instead of ARGN for efficiency