Disambiguate `ialloc::Alignment` from `core::ptr::Alignment`

[MaulingMonkey]

• core::ptr::Alignment is not yet stable
• ialloc::Alignment has better Debug
• core::ptr::Alignment lacks MAX
• ialloc::Alignment lacks log2()
• both lack mask() (as_nonzero()-1)
• align!(16 MiB) ?

[MaulingMonkey]

Can't just re-export, examples will still fail/complain.