async-signature: forward std feature (#1582)

`std` gates the From<std::error::Error> implementation from signature.

This allows to enable the feature without importing signature directly.

async-signature/Cargo.toml

@@ -17,6 +17,7 @@ signature = "=2.3.0-pre.3"
 
 [features]
 digest = ["signature/digest"]
+std = ["signature/std"]
 rand_core = ["signature/rand_core"]
 
 [package.metadata.docs.rs]