Change error message of a failing `assert_eq!`

I'd find it much more valuable if the output would highlight the actual difference. When asserting equality between structs (especially large or nested ones) you often get two blobs of std::fmt::Debug outputs you have to look through to find a (sometimes) one character difference.

Not sure on the feasability of that though [1], but even diff on the output text + color highlighting would be very useful in that situation. Example (inspired by Wordle):

• Red for sections that don't occur in the Debug output of the other side
• Optional: Orange for (larger) sections that are in the other side but not in the same place
• White for matching sections (same place/order)

1. Especially when order does not matter for equality like with HashMap. ↩︎