Fix missing safety in {ptr,NonNull}::as_mut

[DiuDiu777]

In this PR:

1. The initialization precondition for ptr::{ as_mut, as_ref, as_mut_unchecked, as_ref_unchecked } and NonNull::{ as_mut, as_ref } have been moved under the safety contracts.
2. Specified the potential aliasing risks of APIs that return a reference, including ptr::{ as_mut, as_ref, as_mut_unchecked, as_ref_unchecked, as_uninit_mut, as_uninit_ref } and NonNull::{ as_mut, as_ref, as_uninit_mut, as_uninit_ref }. The following code passes the compiler's exclusivity checks, but Miri reports undefined behavior.

fn main() {
    use std::ptr::NonNull;
    let mut x = 0u32;
    let mut ptr = NonNull::new(&mut x).expect("null pointer");
    let x_ref = unsafe { ptr.as_mut() };
    unsafe { 
        *ptr.as_ptr() += 2;  
    }
    *x_ref += 2;
    println!("Final value: {}", x_ref);
}

[rustbot]

r? @Noratrieb

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