space is similar to map space in that it has fixed-size objects.  A
common superclass for a space with fixed size objects is used for the
map space and cell space.

Allocate all cells in cell space.  Handle it during all GCs.  Modify
the free-list node representation (so that the size is not at a fixed
offset in all cells) to allow two-pointer free-list nodes.  Clean up
some stuff in the MC collector.

Review URL: http://codereview.chromium.org/155211

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@2411 ce2b1a6d-e550-0410-aec6-3dcde31c8c00