Rollup merge of #95426 - b-naber:valtrees-slice, r=RalfJung,oli-obk

Include Refs in Valtree Creation

This adds references to `const_to_valtree`, which isn't used in the compiler yet, but after the previous changes we made to the thir and mir representations and this change we should be able to finally introduce them in the next PR.

I wasn't able to properly test this code, except indirectly by including a call of `const_to_valtree` in the code that currently creates constants (`turn_into_const_value`).

r? `@lcnr`

cc `@oli-obk` `@RalfJung`

compiler/rustc_const_eval/src/interpret/place.rs

@@ -191,7 +191,7 @@ impl<'tcx, Tag: Provenance> MPlaceTy<'tcx, Tag> {
     }
 
     #[inline]
-    pub(super) fn len(&self, cx: &impl HasDataLayout) -> InterpResult<'tcx, u64> {
+    pub(crate) fn len(&self, cx: &impl HasDataLayout) -> InterpResult<'tcx, u64> {
         if self.layout.is_unsized() {
             // We need to consult `meta` metadata
             match self.layout.ty.kind() {