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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
