portable-atomic-util: Arc::{into_raw,as_ptr} should not take self

[taiki-e]

It should match with std' APIs.

https://doc.rust-lang.org/nightly/std/sync/struct.Arc.html#method.into_raw

pub fn into_raw(this: Self) -> *const T;
pub fn as_ptr(this: &Self) -> *const T;

https://docs.rs/portable-atomic-util/latest/portable_atomic_util/struct.Arc.html#method.into_raw

pub fn into_raw(self) -> *const T;
pub fn as_ptr(&self) -> *const T;

The current APIs cause Arc::as_ptr to be called in code that tries to call T::as_ptr, so the semantics will change when using std and when using portable-atomic-util.

I was aware that #41 uses a different implementation than std, but I missed the difference in API.