Expose a function to stringify a PanicInfo struct

[Ekleog]

This allows users who want to display the error message generated by console_error_panic_hook outside of the console to do it without having to reimplement javascript-based stack walking themselves.

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Hey! I'm in the process of trying to be nicer to users upon panic, displaying an error page on my yew app instead of just crashing and dumping logs to the console (source). However, I'd rather avoid reimplementing the stack walking that console_error_panic_hook implemented in order to tell the user what they should copy-paste to the issue tracker.

So I thought of just exposing it from console_error_panic_hook, though it may be slightly outside of the expected API surface for it.

WDYT?