Use common formatting for backtraces (#6977)

This makes debug_print_backtrace() use the same formatting as exception
backtraces. The only difference is that the final #{main} is omitted,

Use common formatting for backtraces (#6977)

This makes debug_print_backtrace() use the same formatting as exception
backtraces. The only difference is that the final #{main} is omitted,
because it wouldn't make sense for limited backtraces, and wasn't there
previously either.

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Improve fiber backtraces

The start/resume/throw execute_data is now attached as the prev_execute_data to the bottom frame of the fiber stack when the fiber is running.