They uncovered one bug: in edit/003-shortcuts.mu
<scroll-down> was returning 0 for an address in one place where I
thought it was returning 0 for a boolean.
Now we've eliminated this bad interaction between tangling and punning
literals.
Surprisingly small change, considering how long it took me and how
mind-bending it was. 'return-continuation-until-mark' now behaves like
both call and return instructions, which made it hard to reason about.
