Followup work from making `ValTree` recurse through `ty::Const`

[BoxyUwU]

After #149114 ValTree's Branches variant stores a list of ty::Const instead of a list of ValTrees. See the PR description and associated zulip thread (#project-const-generics > Valtrees that can contain generic params) for a motivation for this.

This has left us with two problems:

1. The name Value is kinda confusing :) These are in some sense not full values. It may be desirable to rename ConstKind::Value, ty::Value, ty::ValTree and ty::ValTreeKind to something different now that they don't represent full values.
2. There are now a bunch of extra Ty in a fully evaluate ValTree. This is likely bad for performance. Though more importantly it means that when CTFE evaluates something to a ValTree it has to pick types for all the nested nodes in a ValTree. This is effectively impossible for CTFE to do correctly as it doesn't have lifetime information so we wind up with a bunch of erased lifetimes entering the type system.

The second point can be observed by looking at debug logs for this code example:

#![feature(adt_const_params, unsized_const_params)]
#![expect(incomplete_features)]
#![crate_type = "lib"]

struct Foo<'a> {
    r: &'a u32,
}

fn bar<const N: Foo<'static>>() { }

fn qux() {
    bar::<{ Foo { r: &1 } }>();
}

0ms DEBUG rustc_trait_selection::traits ct=UnevaluatedConst { def: DefId(0:9 ~ foo[cae9]::qux::{constant#0}), args: [] }
0ms DEBUG rustc_trait_selection::traits return=Ok(ValTree(Branch([ValTree(Leaf(0x00000001): &'{erased} u32)]): Foo::<'static>))

Assigning myself as I'm partially through fixing this second point.

[BoxyUwU]

There are now a bunch of extra Ty in a fully evaluate ValTree

another problem with this is that subtrees of a valtree can have an incompatible type from the parent node's type. while this is expected due to direct lowering, it makes working with valtrees error prone for illformed code as you can no longer easily check "is this an array of u8" as you need to check each subtree for it being a u8 before assuming they're all u8