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/wasmtime/Cargo.toml

@@ -9,8 +9,9 @@ repository = "https://github.com/bytecodealliance/wasmtime"
 readme = "README.md"
 edition = "2018"
 
-[package.metadata.docs.rs]
-rustdoc-args = ["--cfg", "nightlydoc"]
+# FIXME(rust-lang/cargo#9300): uncomment once that lands
+# [package.metadata.docs.rs]
+# rustdoc-args = ["--cfg", "nightlydoc"]
 
 [dependencies]
 wasmtime-runtime = { path = "../runtime", version = "0.25.0" }