Update page slot size computation to take alignment into account (#217)

Fix failed assertion in `WritablePage::add_item_unchecked` when testing
with labeled yfcc dataset. Postgres adds alignment padding if needed
when inserting items to page:


https://github.com/postgres/postgres/blob/0164a0f9ee12e0eff9e4c661358a272ecd65c2d4/src/backend/storage/page/bufpage.c#L304

We need to take this alignment into account when calculating whether an
item will fit. This PR also replaces existing logic in ChainTape with
the new tighter calculation.