assert_eq!() might need a better fail message

[Kimundi]

assert_eq!(left, right) used to fail with fmt!("expected: %?, given: %?", right, left), which was confusing because it was neither clear which of the two parameters of assert_eq is supposed to be the 'given' and 'expected' value, and because the values got printed out swapped compared to the way the arguments are given.

It now fails with fmt!("left: %? != right: %?", left, right), which, while less confusing, is still kinda unclear because it doesn't contain any description of what happened.

The message should probably be more verbose, like fmt!("expected equal, but left side %? != right side %?", left, right)

[emberian]

Visiting for triage; PR as soon as tests are finished running.