Implement cast_slice for raw pointer types

[Paladynee]

Tracking issue: #149103

This PR implements the method cast_slice that defers to the respective slice_from_raw_parts call for *const T, *mut T and NonNull<T>, and copies over their documentation.

impl<T> *const T {
    pub const fn cast_slice(self, len: usize) -> *const [T];
}
impl<T> *mut T {
    pub const fn cast_slice(self, len: usize) -> *mut [T];
}
impl<T> NonNull<T> {
    pub const fn cast_slice(self, len: usize) -> NonNull<[T]>;
}

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[rust-log-analyzer]

The job aarch64-gnu-llvm-20-2 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

 Documenting core v0.0.0 (/checkout/library/core)
error: unresolved link to `cast_slice`
    --> library/core/src/ptr/mut_ptr.rs:1663:46
     |
1663 |     /// Performs the same functionality as [`cast_slice`] on a `*const T`, except that a
     |                                              ^^^^^^^^^^ no item named `cast_slice` in scope
     |
     = help: to escape `[` and `]` characters, add '\' before them like `\[` or `\]`
     = note: `-D rustdoc::broken-intra-doc-links` implied by `-D warnings`
     = help: to override `-D warnings` add `#[allow(rustdoc::broken_intra_doc_links)]`

error: could not document `core`
warning: build failed, waiting for other jobs to finish...
[RUSTC-TIMING] core test:false 25.914
Command `/checkout/obj/build/aarch64-unknown-linux-gnu/stage0/bin/cargo doc --target aarch64-unknown-linux-gnu -Zbinary-dep-depinfo -j 4 -Zroot-dir=/checkout --locked --color always --release -p alloc -p compiler_builtins -p core -p panic_abort -p panic_unwind -p proc_macro -p rustc-std-workspace-core -p std -p std_detect -p sysroot -p test -p unwind --features 'backtrace panic-unwind compiler-builtins-c' --manifest-path /checkout/library/sysroot/Cargo.toml --no-deps --target-dir /checkout/obj/build/aarch64-unknown-linux-gnu/stage1-std/aarch64-unknown-linux-gnu/doc -Zskip-rustdoc-fingerprint -Zrustdoc-map [workdir=/checkout]` failed with exit code 101
Created at: src/bootstrap/src/core/build_steps/doc.rs:781:21
Executed at: src/bootstrap/src/core/build_steps/doc.rs:814:22

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `--stage 2 test --skip tests --skip coverage-map --skip coverage-run --skip library --skip tidyselftest`
Build completed unsuccessfully in 0:30:54
  local time: Wed Nov 19 20:59:39 UTC 2025
  network time: Wed, 19 Nov 2025 20:59:39 GMT
##[error]Process completed with exit code 1.