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/runtime/src/instance/allocator/pooling.rs

@@ -1618,7 +1618,7 @@ mod test {
     }
 
     #[cfg_attr(target_arch = "aarch64", ignore)] // https://github.com/bytecodealliance/wasmtime/pull/2518#issuecomment-747280133
-    #[cfg(all(unix, target_pointer_width = "64"))]
+    #[cfg(all(unix, target_pointer_width = "64", feature = "async"))]
     #[test]
     fn test_stack_zeroed() -> Result<()> {
         let allocator = PoolingInstanceAllocator::new(