Add lifetime to PAddr and VAddr

[nuta]

In #141, I noticed that we need to track the lifetime of a PAddr object which references to OwnedPages. It's problematic because it lead to memory leaks or nasty use-after-free bugs. In this issue, I'll replace PAddr and VAddr as:

#[repr(transparent)]
pub struct PAddr<'a> {
    value: usize,
    _pd: PhantomData<&'a ()>,
}