pub fn exact_len(&self) -> usize {
        self.index
            .iter()
            .filter(|&&i| i != NO_PAGE)
            .map(|index| self.page_table[*index as usize].0.iter().filter(|v| v.is_some()).count())
            .sum()
    }
Get the exact number of addresses in use. This function iterates through each page
and is therefore somewhat expensive.
crates/core/executor/src/memory.rs