Rollup merge of #152148 - Zalathar:tcx-interner, r=petrochenkov

Move `impl Interner for TyCtxt` to its own submodule

This impl is several hundred lines of mostly self-contained, mostly boilerplate code that can be extracted out of the dauntingly large `rustc_middle::ty::context` module.

- The trait and its impl were introduced by #97287.

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There should be no change to compiler behaviour.

Author: JonathanBrouwer