SI-6443 Reuse packSymbols, rather than DIY.

Adriaan poked some holes in the approach to
type substitution. I found what appears to be
a perfect replacement, gift wrapped over in
Typers: packedSymbols.
SI-6443 Expand test coverage with varargs, by-name.

These were already working, due to a serendipitous
ordering of transformations. The tests will keep
it this way.