internal: Revert castable expectation and simplify #14120
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Properly infer types with type casts This PR reenables `Expectation::Castable` (previous attempt at #14104, reverted by #14120) and implements type cast checks, which enable us to infer a bit more. Castable expectations are relatively weak -- they only influence the inference if we cannot infer the types by other means. Therefore, we need to defer possible type unification with the casted type until we type check all expressions of the body. This PR adds a struct and slots in `InferenceContext` for the deferred cast checks (c.f. [`CastCheck`] in `rustc_hir_typeck`). I only implemented the bits that affect the inference result. It should be possible to return type adjustments for well-formed casts and report diagnostics for invalid casts, but I'm leaving them for future work for now. Fixes #11571 Fixes #15246 [`CastCheck`]: https://github.com/rust-lang/rust/blob/da1d099f91ea387a2814a6244dd875a2048b486f/compiler/rustc_hir_typeck/src/cast.rs#L55
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Unfixes #11571, the PR for that introduced some regressions (tried fixing them but doing what rustc does there specifically does not help, probably because something else affects it as well there)