Support slices in type reflection

[BD103]

Tracking issue: #146922

This PR adds support for inspecting slices [T] through type reflection. It does so by adding the new Slice struct + variant, which is very similar to Array but without the len field:

pub struct Slice {
    pub element_ty: TypeId,
}

Here is an example reflecting a slice:

match const { Type::of::<[u8]>() }.kind {
    TypeKind::Slice(slice) => assert_eq!(slice.element_ty, TypeId::of::<u8>()),
    _ => unreachable!(),
}

In addition to this, I also re-arranged the type info unit tests.

r? @oli-obk

[rustbot]

The reflection data structures are tied exactly to the implementation
in the compiler. Make sure to also adjust rustc_const_eval/src/const_eval/type_info.rs

cc @oli-obk

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

[rustbot]

oli-obk is not on the review rotation at the moment.
They may take a while to respond.

[oli-obk]

Needs a rebase. Otherwise lgtm

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.