Replace IsAligned marker struct with const generic in bevy_ptr

[mysteriouslyseeing]

What problem does this solve or what need does it fill?

The Ptr, PtrMut and OwningPtr types use a marker struct to indicate whether the pointer is aligned. It may be more appropriate to use a const generic parameter instead.

What solution would you like?

Replace the Ptr, PtrMut and OwningPtr declarations with variations on the following:

pub struct Ptr<'a, const IS_ALIGNED: bool = true>(NonNull<u8>, PhantomData<&'a u8>);

Perhaps also replace use of the boolean literals true and false with ALIGNED and UNALIGNED for readability (Ptr::<ALIGNED> rather than Ptr::<true>)

pub const ALIGNED: bool = true;
pub const UNALIGNED: bool = false;

Additional context

Supercedes #17988 and thus #18042