clippy: Allow breaking convention to avoid breaking change

[evnu]

This PR disables clippy::wrong-self-convention for two public functions:

error: methods with the following characteristics: (`to_*` and `self` type is `Copy`) usually take `self` by value
   --> rustler/src/types/binary.rs:245:21
    |
245 |     pub fn to_owned(&self) -> Option<OwnedBinary> {
    |                     ^^^^^
    |
    = note: `-D clippy::wrong-self-convention` implied by `-D warnings`
    = help: consider choosing a less ambiguous name
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#wrong_self_convention