Review numeric casts

[robin-nitrokey]

This crate contains many numeric casts. Some of them can be problematic, at least if u32 does not fit in usize on the target platform.

We should therefore review these casts and probably enable the cast_lossless, cast_possible_truncation, cast_possible_wrap, cast_sign_loss lints. (Personally I’d like to enable as_conversions but we often need it for pointer conversions.)