Make serde_derive optional for postcard users

Outside of tests, the only use of serde_derive is in schema.rs, which
is only exposed when postcard's experimental-derive feature is enabled.

Cargo.toml

@@ -40,6 +40,7 @@ default-features = false
 
 [dependencies.serde_derive]
 version = "1"
+optional = true
 
 [dependencies.cobs]
 version = "0.2.3"
@@ -95,7 +96,7 @@ use-crc = ["crc", "paste"]
 # Experimental features!
 #
 # NOT subject to SemVer guarantees!
-experimental-derive = ["postcard-derive", "const_format"]
+experimental-derive = ["postcard-derive", "const_format", "dep:serde_derive"]
 
 [workspace]
 members = ["postcard-derive"]

src/lib.rs

@@ -15,6 +15,7 @@ mod varint;
 
 // Still experimental! Don't make pub pub.
 pub(crate) mod max_size;
+#[cfg(feature = "experimental-derive")]
 pub(crate) mod schema;
 
 /// # Experimental Postcard Features