Rollup merge of #152593 - spirali:valtreekind-list, r=lcnr

Box in `ValTreeKind::Branch(Box<[I::Const]>)` changed to `List`

This is related to trait system refactoring. It fixes the FIXME in `ValTreeKind`

```
   // FIXME(mgca): Use a `List` here instead of a boxed slice
    Branch(Box<[I::Const]>),
```

It introduces `Interner::Consts`, changes `Branch(Box<[I::Const]>)` to `Branch(I::Consts)`, and updates all relevant places.

r? lcnr

Author: JonathanBrouwer