Instead of passing in a tape to print in case we need in, pass in a trap that
can be called to generate the tape on-demand.
Eagerly rendering printouts was costing us a lot of time when we didn't actually
need them, because the pretty-printer is slow.
Events always pass through these, adding to the stack trace on-crash.
This information is practically never useful, however. Adding !. leaves
these cores out of the traces.
