Bump allocate downward about book HOT 3 CLOSED

I'd love this to be a "good first issue" but it does require understanding of Immix and the subject matter in general. It's not such a small issue either due to the need to update the book.

from book.

Additionally, as the book chapter in alignment states, objects should be aligned on double-word boundaries. The object header should be aligned on a word boundary. These constraints are not correctly implemented yet and should be as part of this issue.

from book.

Additionally number 2: the metadata should be inside the memory block, not a separately allocated data structure. I originally did that for easier type management.

from book.