Type of some division expressions is ambiguous and the default interpretation is bad

[marcoeilers]

Gobra frequently generates permission amounts like 1 / 4000 / 2 for SCION code by @jcp19. These are intended to be a fractional permission 1/4000 divided by two, i.e., the permission value 1/8000. Instead, they are apparently parsed as an integer division 1 \ 4000, which is zero, which is then the numerator in the fractional permission 0 / 2. This is bad because a) it's not the intended meaning, and b) there is no way to pretty-print it so that the parser returns the intended meaning.