grm: add support for converting args

[kou]

Before:

args = GRM.args_new
GRM.args_push(args, 'kind', 's', :const_string, 'plot3')
GRM.args_push(args, 'x', 'nD', :int, n, :voidp, x.pack('d*'))
GRM.args_push(args, 'y', 'nD', :int, n, :voidp, y.pack('d*'))
GRM.args_push(args, 'z', 'nD', :int, n, :voidp, z.pack('d*'))
GRM.plot(args)
gets
GRM.args_delete(args)

After:

GRM.plot(kind: "plot3",
         x: x,
         y: y,
         z: z)
gets