Lint against &T to &mut T and &T to &UnsafeCell<T> transmutes

[ChayimFriedman2]

This adds the lint against &T->&UnsafeCell<T> transmutes, and also check in struct fields.

The code is quite complex; I've tried my best to simplify and comment it.

This is missing one parts: array transmutes. When transmuting an array, this only consider the first element. The reason for that is that the code is already quite complex, and I didn't want to complicate it more. This catches the most common pattern of transmuting an array into an array of the same length with type of the same size; more complex cases are likely not properly handled. We could take a bigger sample, for example the first and last elements to increase the chance that the lint will catch mistakes, but then the runtime complexity becomes exponential with the nesting of the arrays ([[[[[T; 2]; 2]; 2]; 2]; 2] has complexity of O(2**5), for instance).

Fixes #111229.

I've made this a deny-by-default and the wording says it's immediate UB, but from what I understand, under Tree Borrows this is not UB? CC rust-lang/unsafe-code-guidelines#303. So maybe we should make this warn-by-default and make the wording less strong. But #111229 says it causes miscompilations... So I'm not sure what is really the situation.

This also doesn't handle other ways of doing the transmute, such as via pointer casts. A fix for that should be quite simple, but I left it out for future PR.