RawTable::allocation_info returns non zero size for a fresh empty map

[koute]

The following code:

#[test]
fn test() {
    assert_eq!(hashbrown::raw::RawTable::<()>::new().allocation_info().1.size(), 0);
}

fails with:

thread 'test' panicked at 'assertion failed: `(left == right)`
  left: `17`,
 right: `0`', src/lib.rs:3:5

Since RawTable::new doesn't allocate any memory I'd expect this to return 0.

[Amanieu]

cc @stepancheg