Don't copy single-use strings to the zero page

[mysterymath]

The compiler is currently allocating strings that are only printed once to the zero page. It's likely overestimating the loop count for accesses to the strings; it should never pay off to move such to the zero page unless the string is used several times, since the loop that would copy it there is the same size as the loop that accesses it. We either need to add a special-case heuristic or expose better loop trip count information to the zero page allocation pass.