add SliceIndex wrapper types Last and Clamp<Idx>

[Qelxiros]

Tracking issue: #146179

[rust-log-analyzer]

The job aarch64-gnu-llvm-19-1 failed! Check out the build log: (web) (plain enhanced) (plain)

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    |
    = note: `-D missing-docs` implied by `-D warnings`
    = help: to override `-D warnings` add `#[allow(missing_docs)]`

error: type does not implement `fmt::Debug`; consider adding `#[derive(Debug)]` or a manual implementation
 --> library/core/src/index.rs:6:1
  |
6 | pub struct Clamp<Idx>(pub Idx);
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |
  = note: `-D missing-debug-implementations` implied by `-D warnings`
  = help: to override `-D warnings` add `#[allow(missing_debug_implementations)]`

error: missing documentation for a struct
 --> library/core/src/index.rs:6:1
  |
6 | pub struct Clamp<Idx>(pub Idx);
  | ^^^^^^^^^^^^^^^^^^^^^

error: type does not implement `fmt::Debug`; consider adding `#[derive(Debug)]` or a manual implementation
 --> library/core/src/index.rs:9:1
  |
9 | pub struct Last;
  | ^^^^^^^^^^^^^^^^