async-signature: move to AFIT

`AFIT` is expected in the upcoming rust-1.75 release (scheduled for
2023/12/28).

`#[allow(async_fn_in_trait)]` is required until a solution to RPITIT is
merged in rust.
This put responsability on the implementor of `async_signature::AsyncSigner`
to make sure their future is `Send`able.
see rust-lang/rust#115822 (comment)
for more context.

async-signature/Cargo.toml

@@ -10,7 +10,7 @@ readme        = "README.md"
 keywords      = ["crypto", "ecdsa", "ed25519", "signature", "signing"]
 categories    = ["cryptography", "no-std"]
 edition       = "2021"
-rust-version  = "1.60"
+rust-version  = "1.75"
 
 [dependencies]
 async-trait = "0.1.9"

async-signature/src/lib.rs

@@ -17,13 +17,12 @@ pub use signature::{self, Error};
 #[cfg(feature = "digest")]
 pub use signature::digest::{self, Digest};
 
-use async_trait::async_trait;
 
 /// Asynchronously sign the provided message bytestring using `Self`
 /// (e.g. client for a Cloud KMS or HSM), returning a digital signature.
 ///
 /// This trait is an async equivalent of the [`signature::Signer`] trait.
-#[async_trait(?Send)]
+#[allow(async_fn_in_trait)]
 pub trait AsyncSigner<S: 'static> {
     /// Attempt to sign the given message, returning a digital signature on
     /// success, or an error if something went wrong.
@@ -33,7 +32,6 @@ pub trait AsyncSigner<S: 'static> {
     async fn sign_async(&self, msg: &[u8]) -> Result<S, Error>;
 }
 
-#[async_trait(?Send)]
 impl<S, T> AsyncSigner<S> for T
 where
     S: 'static,
@@ -48,7 +46,7 @@ where
 ///
 /// This trait is an async equivalent of the [`signature::DigestSigner`] trait.
 #[cfg(feature = "digest")]
-#[async_trait(?Send)]
+#[allow(async_fn_in_trait)]
 pub trait AsyncDigestSigner<D, S>
 where
     D: Digest + 'static,
@@ -60,7 +58,6 @@ where
 }
 
 #[cfg(feature = "digest")]
-#[async_trait(?Send)]
 impl<D, S, T> AsyncDigestSigner<D, S> for T
 where
     D: Digest + 'static,