`abs` should add a case for `(-> Fixnum Nonnegative-Fixnum)`

[LiberalArtist]

See https://github.com/racket/typed-racket/issues/1309 for a motivating example.

[sorawee]

One thing that should be looked out for is that (most-positive-fixnum) is less than (abs (most-negative-fixnum)) by one.

[samth]

Yes, as @sorawee points out this is unfortunately not true:

> (fixnum? (abs (most-negative-fixnum)))
#f