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

cranelift/codegen/src/isa/x64/inst/args.rs

@@ -462,6 +462,7 @@ pub(crate) enum InstructionSet {
 
 /// Some SSE operations requiring 2 operands r/m and r.
 #[derive(Clone, Copy, PartialEq)]
+#[allow(dead_code)] // some variants here aren't used just yet
 pub enum SseOpcode {
     Addps,
     Addpd,