Unsafe ref_cast_custom function emits unused_unsafe lint on toolchains older than 1.65

[dtolnay]

use ref_cast::{ref_cast_custom, RefCastCustom};

#[derive(RefCastCustom)]
#[repr(transparent)]
pub struct Repro(u8);

impl Repro {
    #[ref_cast_custom]
    pub unsafe fn unchecked_ref_cast_mut(x: &mut u8) -> &mut Self;
}

warning: unnecessary `unsafe` block
 --> src/main.rs:9:66
  |
9 |     pub unsafe fn unchecked_ref_cast_mut(x: &mut u8) -> &mut Self;
  |     -------------------------------------------------------------^ unnecessary `unsafe` block
  |     |
  |     because it's nested under this `unsafe` fn
  |
  = note: `#[warn(unused_unsafe)]` on by default
  = note: this `unsafe` block does contain unsafe operations, but those are already allowed in an `unsafe fn`
  = note: `#[allow(unsafe_op_in_unsafe_fn)]` on by default

The warning does not appear anymore on 1.65 (current beta) and up, but we might as well stick an allow(unused_unsafe) into the generated code.