Fix compiling in nightly

[kerosina]

In nightly, the compiler complains about the box_pointers lint:

error: lint `box_pointers` has been removed: it does not detect other kinds of allocations, and existed only for historical reasons
  --> src\cast\cast_channel.rs:12:10
   |
12 | #![allow(box_pointers)]
   |

It was removed 4 months ago.
This PR removes this lint from both cast_channel.rs and authority_keys.rs.

[azasypkin]

Hey @kerosina,

Thanks for the PR! I see the CI is failing for some reason, I'll check it out later this weekend and merge your PR.