Auto merge of #107924 - eggyal:move_fold_visit_traits_to_type_lib_with_trait_alias, r=oli-obk

Move folding & visiting traits into type library

This is a rework of #107712, following feedback on that PR.

In particular, this version uses trait aliases to reduce the API churn for trait consumers.  Doing so requires a workaround for #107747 until its fix in #107803 is merged into the stage0 compiler; this workaround, which uses conditional compilation based on the `bootstrap` configuration predicate, sits in dedicated commit b409329c for ease of reversion.

The possibility of the `rustc_middle` crate retaining its own distinct versions of each folding/visiting trait, blanket-implemented on all types that implement the respective trait in the type library, was also explored: however since this would necessitate making each `rustc_middle` trait a subtrait of the respective type library trait (so that such blanket implementations can delegate their generic methods), no benefit would be gained.

r? types

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