Auto merge of #32064 - bluss:str-slice-panic, r=alexcrichton

Auto merge of #32064 - bluss:str-slice-panic, r=alexcrichton

Fix panic on string slicing error to truncate the string

The string may be arbitrarily long, but we want to limit the panic
message to a reasonable length. Truncate the string if it is too long
(simply to char boundary).

Also add details to the start <= end message. I think it's ok to flesh
out the code here, since it's in a cold function.

Fixes #32063