Function: _tovec
Class: gp2c_internal
Help: Create a vector holding the arguments (shallow)
Description:
 (gen):vec               mkvec($1)
 (gen,gen):vec           mkvec2($1, $2)
 (gen,gen,gen):vec       mkvec3($1, $2, $3)
 (gen,gen,gen,gen):vec   mkvec4($1, $2, $3, $4)
 (gen,...):vec           mkvecn($#, $2)

