Constant mmtk::policy::markcompactspace::GC_EXTRA_HEADER_WORD

pub const GC_EXTRA_HEADER_WORD: usize = 1;

For each MarkCompact object, we need one extra word for storing forwarding pointer (Lisp-2 implementation). Note that considering the object alignment, we may end up allocating/reserving more than one word per object. See MarkCompactSpace::HEADER_RESERVED_IN_BYTES.