Change error message of a failing `assert_eq!`

Changing the message will not be an effective way of eliminating inconsistency in the ecosystem: most people will not notice, so we'll just be making code worse.

If this is something that could be linted on, perhaps, but otherwise this is not a good idea. There is clearly ambiguity in the ecosystem, you cannot just force consensus through a behavior change.

Furthermore, not every assertion case has a clear expected/actual, sometimes the thing is indeed just "are these two things the same".

A better proposal that helps eliminate confusion would be to add expected= and actual= disambiguators to the macro, e.g. assert_eq!(expected = foo, actual = bar), which change the displayed error message. I'm not sure if it pulls its weight (it feels verbose to me), but it would allow for folks to opt in to expected/actual messaging. I certainly have times where I wish the assertion error message told me which was the expected value.