Add integer truncation and extension methods

[joshtriplett]

Tracking issue: #154330

This provides .truncate(), .saturating_truncate(), .checked_truncate(), and .extend().

These only work within the same signedness (use .cast_signed() and .cast_unsigned() to change sign).

The truncation methods only work to smaller (or equal) types. .extend() only works to larger (or equal) types.

For the purposes of truncation and extending, u128 is considered larger than or equal to the size of usize, and usize is considered larger than u16 or u8. This is consistent with From/Into conversions. We might, in the future, want to consider ways to expand this.

Much of this was pair-programmed with @Amanieu.

In order to seal the new traits, this PR also adds a core::sealed::Sealed, like the one in std. I didn't modify std to re-export the same one, since by definition it isn't nameable, and since doing that would require that it be nameable (even if it was #[unstable]).

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[rust-log-analyzer]

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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

Executing "/scripts/stage_2_test_set1.sh"
+ /scripts/stage_2_test_set1.sh
PR_CI_JOB set; skipping tidy
+ '[' 1 == 1 ']'
+ echo 'PR_CI_JOB set; skipping tidy'
+ SKIP_TIDY='--skip tidy'
+ ../x.py --stage 2 test --skip tidy --skip compiler --skip src
##[group]Building bootstrap
    Finished `dev` profile [unoptimized] target(s) in 0.04s
##[endgroup]
downloading https://static.rust-lang.org/dist/2026-03-05/rustfmt-nightly-aarch64-unknown-linux-gnu.tar.xz
---
10    = help: items from traits can only be used if the trait is in scope
11 help: trait `SettingsExt` which provides `ext` is implemented but not in scope; perhaps you want to import it

12    |
13 LL + use auto::SettingsExt;
14    |
+ help: there is a method `extend` with a similar name
+    |
+ LL |     a.extend();
+    |          +++
---
To only update this specific test, also pass `--test-args imports/issue-114682-3.rs`

error: 1 errors occurred comparing output.
status: exit status: 1
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/imports/issue-114682-3.rs" "-Zthreads=1" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/imports/issue-114682-3" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-L" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/imports/issue-114682-3/auxiliary"
stdout: none
--- stderr -------------------------------
error[E0599]: no method named `ext` found for type `u8` in the current scope
##[error]  --> /checkout/tests/ui/imports/issue-114682-3.rs:20:7
   |