ash: Enable mostly-unused hint to speed up compilation

Rust recently introduced the `mostly-unused` hint to speed up
compilation of large crates where a significant portion of the
contents are unlikely to be used concurrently.  `ash` is such a crate,
which takes quite long to compile relative to the amount and kind of
abstraction it provides, simply caused by the vastness of the Vulkan
API (and possible remaining inefficiencies in our generated code).  This
hint helps the compiler defer code generation until it knows it is
definitely referenced and used, much like generics and `#[inline]` (the
latter is already used consistently across this crate).  All in all this
new hint reduces compile time of the `ash` crate by about 8%.

It however requires the use of a `nightly` compiler and passing of the
`-Zprofile-hint-mostly-unused` flag.  The newly introduced `[hints]`
table is not a syntax or parsing error on older compilers, but does
issue a matching `unused manifest key: hints` warning.

https://blog.rust-lang.org/inside-rust/2025/07/15/call-for-testing-hint-mostly-unused/

Author: MarijnS95

ash/Cargo.toml

@@ -44,3 +44,6 @@ no-dev-version = true
 [package.metadata.docs.rs]
 all-features = true
 rustdoc-args = ["--cfg", "docsrs"]
+
+[hints]
+mostly-unused = true