fix: add explicit prelude imports for wasm32v1-none no_std + alloc

When building serde_core for wasm32v1-none target with no_std but with
the alloc feature enabled, the Rust compiler doesn't automatically inject
the prelude. This causes ?Sized bounds to fail with "bound modifier ?
can only be applied to Sized" errors.

This commit adds explicit prelude imports to all modules that:
1. Use `use crate::lib::*;` for serde's internal lib facade
2. Use `?Sized` bounds in their code

The fix adds:
```rust
#[allow(unused_imports)]
#[cfg(not(feature = "std"))]
use ::core::prelude::rust_2021::*;
```

This ensures Sized, Clone, Copy, and other prelude traits are in scope
for the ?Sized syntax to work correctly on wasm32v1-none targets.

Affected files:
- serde_core/src/de/impls.rs
- serde_core/src/de/mod.rs
- serde_core/src/private/doc.rs
- serde_core/src/ser/fmt.rs
- serde_core/src/ser/impls.rs
- serde_core/src/ser/impossible.rs
- serde_core/src/ser/mod.rs

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

serde_core/src/de/impls.rs

@@ -1,5 +1,11 @@
 use crate::lib::*;
 
+// Explicit prelude import for wasm32v1-none target compatibility
+// These may appear unused but are required for ?Sized bounds on wasm32v1-none
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 use crate::de::{
     Deserialize, Deserializer, EnumAccess, Error, MapAccess, SeqAccess, Unexpected, VariantAccess,
     Visitor,

serde_core/src/de/mod.rs

@@ -114,6 +114,11 @@
 
 use crate::lib::*;
 
+// Explicit prelude import for wasm32v1-none target compatibility
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 ////////////////////////////////////////////////////////////////////////////////
 
 pub mod value;

serde_core/src/private/doc.rs

@@ -2,6 +2,11 @@
 
 use crate::lib::*;
 
+// Explicit prelude import for wasm32v1-none target compatibility
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 use crate::ser;
 
 #[doc(hidden)]

serde_core/src/ser/fmt.rs

@@ -1,4 +1,10 @@
 use crate::lib::*;
+
+// Explicit prelude import for wasm32v1-none target compatibility
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 use crate::ser::{Error, Impossible, Serialize, Serializer};
 
 impl Error for fmt::Error {

serde_core/src/ser/impls.rs

@@ -1,5 +1,11 @@
 use crate::lib::*;
 
+// Explicit prelude import for wasm32v1-none target compatibility
+// These may appear unused but are required for ?Sized bounds on wasm32v1-none
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 use crate::ser::{Error, Serialize, SerializeTuple, Serializer};
 
 ////////////////////////////////////////////////////////////////////////////////

serde_core/src/ser/impossible.rs

@@ -2,6 +2,11 @@
 
 use crate::lib::*;
 
+// Explicit prelude import for wasm32v1-none target compatibility
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 use crate::ser::{
     self, Serialize, SerializeMap, SerializeSeq, SerializeStruct, SerializeStructVariant,
     SerializeTuple, SerializeTupleStruct, SerializeTupleVariant,

serde_core/src/ser/mod.rs

@@ -109,6 +109,11 @@
 
 use crate::lib::*;
 
+// Explicit prelude import for wasm32v1-none target compatibility
+#[allow(unused_imports)]
+#[cfg(not(feature = "std"))]
+use ::core::prelude::rust_2021::*;
+
 mod fmt;
 mod impls;
 mod impossible;