Space::is_in is implemented the same by all Space subclasses, move it to Space and make it non-virtual.
Space::used_region has a bogus implementation that is never useful, remove it.
Consolidate more code related to saved_marks in the Space base class
Make CompactibleSpace::reset_after_compaction a pure virtual.
Give Space::minimum_free_block_size a default implementation since only CMS needs to override it.
This is useful in order to reduce the amount of code needed to disconnect G1 from ContiguousSpace.