Revenge of the gradual guarantee: 'Show casts' setting changes stepper semantics

[disconcision]

This might be derivative the issues addressed by #1453 but the outer behavior seems distinct.

With show casts off, i was noticing the many function evaluations were getting stuck in the stepper:

which i reduced to:

(which works fine without the hole annotation on the x)

if you turn 'show casts' on, you can resume stepping.

@Negabinary