Use stable Rust on CI to test the x64 backend (#2766)

* Use stable Rust on CI to test the x64 backend

This commit leverages the newly-released 1.51.0 compiler to test the
new backend on Windows and Linux with a stable compiler instead of a
nightly compiler. This isolates the nightly build to just the nightly
documentation generation and fuzzing, both of which rely on nightly for
the best results right now.

* Use updated stable in book build job

* Run rustfmt for new stable

* Silence new warnings for wasi-nn

* Allow some dead code in the x64 backend

Looks like new rustc is better about emitting some dead-code warnings

* Update rust in peepmatic job

* Fix a test in the pooling allocator

* Remove `package.metdata.docs.rs` temporarily

Needs resolution of https://github.com/rust-lang/cargo/pull/9300 first

* Fix a warning in a wasi-nn example

crates/wasi-nn/examples/classification-example/src/main.rs

@@ -49,7 +49,8 @@ pub fn main() {
             0,
             &mut output_buffer[..] as *mut [f32] as *mut u8,
             (output_buffer.len() * 4).try_into().unwrap(),
-        );
+        )
+        .unwrap();
     }
     println!(
         "Found results, sorted top 5: {:?}",

crates/wasi-nn/examples/wasi-nn-rust-bindings/src/generated.rs

@@ -152,6 +152,7 @@ pub unsafe fn compute(context: GraphExecutionContext) -> Result<()> {
     }
 }
 
+#[allow(improper_ctypes)]
 pub mod wasi_ephemeral_nn {
     use super::*;
     #[link(wasm_import_module = "wasi_ephemeral_nn")]