Rollup merge of #139515 - compiler-errors:sig-mismatch, r=lcnr

Improve presentation of closure signature mismatch from `Fn` trait goal

Flip the order of "expected" and "found" since that wasn't correct.

Don't present the arguments as a tuple, since it leaves a trailing comma. Instead, just use `fn(arg, arg)`.

Finally, be better with binders since we were just skipping binders.

r? oli-obk or reassign

Author: matthiaskrgr