Use AssertKind::fmt_assert_args now that it's public

[dario23]

See #467 and rust-lang/rust#84392 for some context.

[dario23]

hm, currently fails because the messages change in the tests.. and looking at the differences, i think we might want to keep using the Display impl as far as possible still, as the messages produced by fmt_assert_args are not the same as the Display impl, e.g.

// .fmt_assert_args()
msg: "7:9: 7:14: [Prusti: verification error] assertion might fail with \"\"attempt to divide `{}` by zero\", _4\""

vs.

// .description()
msg: "assertion might fail with \"attempt to divide by zero\"",