Add more array references to raw pointer casts 2.

[jagourq]

Adds &mut [T; n] to *mut T and &mut [T; n] to *const T casts. Closes #24151 and resubmits the changes made in #31418.

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[Aatch]

The original issue doesn't mention &mut [T; n] -> *const T casts.

@nrc, @alexcrichton is that ok? Note that &mut T -> *const T works as a cast (the &mut T is coearced to a &T first).

[nrc]

In general, casting is not transitive - A -> B -> C does not imply A -> C. In this case, I think the &mut [T; n] -> *const T is OK. I see no reason to disallow it, at leasy.

[alexcrichton]

I'm not sure whether this was intended to be allowed, but I think there was an RFC on it?

[jagourq]

One thing I did notice when writing tests for the change was the file vector-cast-weirdness.rs checked to see that casts from &mut [T; n] to *mut T failed. It also checked for something that is now allowed which seemed strange.

[alexcrichton]

Closing due to inactivity, but feel free to resubmit!

[jagourq]

@alexcrichton are there any changes I should make before resubmitting?