Add safe constructors for untyped pointers `Ptr` and `PtrMut`

[joseph-gio]

Objective

Currently, Ptr and PtrMut can only be constructed via unsafe code. This means that downgrading a reference to an untyped pointer is very cumbersome, despite being a very simple operation.

Solution

Define conversions for easily and safely constructing untyped pointers. This is the non-owned counterpart to OwningPtr::make.

Before:

let ptr = unsafe { PtrMut::new(NonNull::from(&mut value).cast()) };

After:

let ptr = PtrMut::from(&mut value);

[DJMcNab]

This looks reasonable to me, but I've added S-Controversial so it doesn't get merged before @TheRawMeatball has given their input.

[cart]

bors r+

[bors[bot]]

Pull request successfully merged into main.

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