Make Kani reject mutable pointer casts if padding is incompatible and memory initialization is checked #3332
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This PR introduces layout checks for types to instrument mutable pointer casts. If two types have incompatible padding (e.g. a padding byte in one is a data byte in the other or vice-versa), an "unsupported check" assertion is inserted. This overapproximates for soundness, since the casts do not cause UB themselves, but an alternative solution involves tracking every MIR place, which is costly.
Resolves #3324
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