Ensure PAULI-SUM is treating the AST with respect

[ecpeterson]

In the course of writing #518, I had to tweak some of the routines that process memory references / delayed expressions. I suspected that #498, which introduced AS PAULI-SUM, had some AST abuses that escaped notice, and I think those started to resurface in #518.

Double back and re-investigate the AST structure around these gate definitions and applications, both as they're parsed and as they're compiled.