Auto merge of #95643 - WaffleLapkin:ptr_convenience, r=joshtriplett

Auto merge of #95643 - WaffleLapkin:ptr_convenience, r=joshtriplett

Add convenience byte offset/check align functions to pointers

This PR adds the following APIs:
```rust
impl *const T {
    // feature gates `pointer_byte_offsets` and `const_pointer_byte_offsets
    pub const unsafe fn byte_offset(self, count: isize) -> Self;
    pub const fn wrapping_byte_offset(self, count: isize) -> Self;
    pub const unsafe fn byte_offset_from(self, origin: *const T) -> isize;
    pub const unsafe fn byte_add(self, count: usize) -> Self;
    pub const unsafe fn byte_sub(self, count: usize) -> Self;
    pub const fn wrapping_byte_add(self, count: usize) -> Self;
    pub const fn wrapping_byte_sub(self, count: usize) -> Self;

    // feature gate `pointer_is_aligned`
    pub fn is_aligned(self) -> bool where T: Sized;
    pub fn is_aligned_to(self, align: usize) -> bool;
}
// ... and the same for` *mut T`
```

Note that all functions except `is_aligned` do **not** require `T: Sized` as their pointee-sized-offset counterparts.

cc `@oli-obk` (you may want to check that I've correctly placed `const`s)
cc `@RalfJung`