Support slices in type reflection
Tracking issue: rust-lang/rust#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:
```rust
pub struct Slice {
pub element_ty: TypeId,
}
```
Here is an example reflecting a slice:
```rust
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
a4cf93bccc