Bug#1141806: rust-prio: FTBFS: error: failed to select a version for the requirement `hex-literal = "^0.4.1"`

Hi,

I am looking at hex-literal regressions[1], and prepared a fix to your pack=
age.
I tried creating a merge request, but the salsa repo does not allow it.

Please, pull from my fork if you agree with the changes.
My fork:
https://salsa.debian.org/rcpassos/rust-prio

I relaxed the hex-literal dependency.
[1] https://tracker.debian.org/pkg/rust-hex-literal

Thanks,
Rafael Passos