ecdsa: simplify serde feature (#874)

To get `serde` support for `VerifyingKey`, confusingly the `pem` feature
previously needed to be enabled (even though PEM is not actually used).

This changed the `serde` feature to auto-enable `pkcs8`, similar to
what was done in the `elliptic-curve` crate.

ecdsa/Cargo.toml

@@ -44,7 +44,7 @@ digest = ["dep:digest", "signature/digest"]
 hazmat = []
 pkcs8 = ["digest", "elliptic-curve/pkcs8", "der"]
 pem = ["elliptic-curve/pem", "pkcs8"]
-serde = ["elliptic-curve/serde", "serdect"]
+serde = ["elliptic-curve/serde", "pkcs8", "serdect"]
 signing = ["arithmetic", "digest", "hazmat", "rfc6979"]
 verifying = ["arithmetic", "digest", "hazmat"]
 

ecdsa/src/verifying.rs

@@ -37,6 +37,9 @@ use elliptic_curve::pkcs8::{
     AssociatedOid, ObjectIdentifier,
 };
 
+#[cfg(feature = "serde")]
+use serdect::serde::{de, ser, Deserialize, Serialize};
+
 #[cfg(feature = "sha2")]
 use {
     crate::{
@@ -48,9 +51,6 @@ use {
 #[cfg(all(feature = "alloc", feature = "pkcs8"))]
 use elliptic_curve::pkcs8::EncodePublicKey;
 
-#[cfg(all(feature = "pem", feature = "serde"))]
-use serdect::serde::{de, ser, Deserialize, Serialize};
-
 /// ECDSA public key used for verifying signatures. Generic over prime order
 /// elliptic curves (e.g. NIST P-curves).
 ///
@@ -453,7 +453,7 @@ where
     }
 }
 
-#[cfg(all(feature = "pem", feature = "serde"))]
+#[cfg(feature = "serde")]
 impl<C> Serialize for VerifyingKey<C>
 where
     C: EcdsaCurve + AssociatedOid + CurveArithmetic + PointCompression,
@@ -468,7 +468,7 @@ where
     }
 }
 
-#[cfg(all(feature = "pem", feature = "serde"))]
+#[cfg(feature = "serde")]
 impl<'de, C> Deserialize<'de> for VerifyingKey<C>
 where
     C: EcdsaCurve + AssociatedOid + CurveArithmetic + PointCompression,