Rollup merge of #117586 - compiler-errors:the-canonicalizer, r=lcnr

Uplift the (new solver) canonicalizer into `rustc_next_trait_solver`

Uplifts the new trait solver's canonicalizer into a new crate called `rustc_next_trait_solver`.

The crate name is literally a bikeshed-avoidance name, so let's not block this PR on that -- renames are welcome later.

There are a host of other changes that were required to make this possible:
* Expose a `ConstTy` trait to get the `Interner::Ty` from a `Interner::Const`.
* Expose some constructor methods to construct `Bound` variants. These are currently methods defined on the interner themselves, but they could be pulled into traits later.
* Expose a `IntoKind` trait to turn a `Ty`/`Const`/`Region` into their corresponding `*Kind`s.
* Some minor tweaks to other APIs in `rustc_type_ir`.

The canonicalizer code itself is best reviewed **with whitespace ignored.**

r? ``@lcnr``

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