Function prune_size

fn prune_size<'a>(
    indexes: impl Iterator<Item = &'a PageKey>,
    file_size: u64,
    page_size: u64,
) -> u64

Prunes the size of the last page based on the indexes. We have following cases:

1. The rest file size is less than the page size, read to the end of the file.
2. Otherwise, read the page size.