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/jit/src/unwind/systemv.rs

@@ -90,7 +90,10 @@ impl UnwindRegistry {
         let mut eh_frame = EhFrame(EndianVec::new(RunTimeEndian::default()));
         table.write_eh_frame(&mut eh_frame).unwrap();
 
-        if cfg!(any(all(target_os = "linux", target_env = "gnu"), target_os = "freebsd")) {
+        if cfg!(any(
+            all(target_os = "linux", target_env = "gnu"),
+            target_os = "freebsd"
+        )) {
             // libgcc expects a terminating "empty" length, so write a 0 length at the end of the table.
             eh_frame.0.write_u32(0).unwrap();
         }
@@ -101,7 +104,10 @@ impl UnwindRegistry {
     }
 
     unsafe fn register_frames(&mut self) {
-        if cfg!(any(all(target_os = "linux", target_env = "gnu"), target_os = "freebsd")) {
+        if cfg!(any(
+            all(target_os = "linux", target_env = "gnu"),
+            target_os = "freebsd"
+        )) {
             // On gnu (libgcc), `__register_frame` will walk the FDEs until an entry of length 0
             let ptr = self.frame_table.as_ptr();
             __register_frame(ptr);