Properly infer types with type casts

[lowr]

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

[lowr]

I'm dumping analysis-stats results below. TL;DR is that none of the numbers regressed and several ?ty numbers improved a bit. All crates besides self are from rustc-perf@c52ee6 as usual.

crate	75ac37f	this branch
self (75ac37f)	exprs: 605528, ??ty: 97, ?ty: 171, !ty: 3 pats: 151212, ??ty: 20, ?ty: 8, !ty: 0	exprs: 605528, ??ty: 97, ?ty: 168, !ty: 3 pats: 151212, ??ty: 20, ?ty: 8, !ty: 0
diesel	exprs: 161838, ??ty: 305, ?ty: 4900, !ty: 63 pats: 33418, ??ty: 122, ?ty: 1211, !ty: 0	exprs: 161838, ??ty: 305, ?ty: 4900, !ty: 63 pats: 33418, ??ty: 122, ?ty: 1211, !ty: 0
webrender/webrender	exprs: 131728, ??ty: 93, ?ty: 42, !ty: 2 pats: 24082, ??ty: 13, ?ty: 15, !ty: 11	exprs: 131728, ??ty: 92, ?ty: 40, !ty: 2 pats: 24082, ??ty: 13, ?ty: 15, !ty: 11
ripgrep	exprs: 72427, ??ty: 27, ?ty: 7, !ty: 0 pats: 12079, ??ty: 3, ?ty: 0, !ty: 0	exprs: 72427, ??ty: 27, ?ty: 7, !ty: 0 pats: 12079, ??ty: 3, ?ty: 0, !ty: 0

[HKalbasi]

@bors r+

[bors]

📌 Commit 074488b has been approved by HKalbasi

It is now in the queue for this repository.

[bors]

⌛ Testing commit 074488b with merge 037844c...

[bors]

☀️ Test successful - checks-actions
Approved by: HKalbasi
Pushing 037844c to master...