fix: introduce codec feature flag

Implementing the `Codec` trait needs at least Rust version 1.75. This commit
introduces a feature flag, so that lower versions of Rust can be supported,
in case the `Codec` trait implementation is not needed.

Cargo.toml

@@ -26,7 +26,8 @@ serde_bytes = { version = "0.11.9", default-features = false, features = ["alloc
 serde-transcode = "1.1.1"
 
 [features]
-default = ["std"]
+default = ["codec", "std"]
 std = ["cbor4ii/use_std", "ipld-core/std", "serde/std", "serde_bytes/std"]
+codec = ["ipld-core/codec"]
 # Prevent deserializing CIDs as bytes as much as possible.
 no-cid-as-bytes = []

src/lib.rs

@@ -117,7 +117,7 @@ extern crate alloc;
 mod cbor4ii_nonpub;
 // The `Codec` implementation is only available if the `no-cid-as-bytes` feature is disabled, due
 // to the links being extracted with a Serde based approach.
-#[cfg(all(feature = "std", not(feature = "no-cid-as-bytes")))]
+#[cfg(all(feature = "std", not(feature = "no-cid-as-bytes"), feature = "codec"))]
 pub mod codec;
 pub mod de;
 pub mod error;