Panic messages could be more helpful

[grothesque]

(For sure the following is luxury, but I'm noting it here nevertheless.)

Currently, panic messages are very terse. Debugging often requires inserting some dbg! invocations. For example mapping.rs has:

assert!(index < self.rank(), "invalid dimension");

I think that it would be helpful if the above was replaced by, for example,

assert!(index < self.rank(), "Expecting a dimension smaller than {}, but got {}.", self.rank(), index);