Use None::<u64> instead of 0 to denote an absence of value

[samueltardieu]

clippy_lints::casts::utils::int_ty_to_nbits used to return 0 to indicate that the argument was not an integral type. It is more idiomatic to use Option<u64> as a return type.

Also, I've added a few FIXME: at places where only f32 and f64 are considered, as we now have f16 and f128 to deal with.

changelog: none

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[llogiq]

Thank you!