Data structure to reference a MMTk 4 MB chunk.
A byte-map to record all the allocated chunks.
A plan can use this to maintain records for the chunks that they used, and the states of the chunks.
Any plan that uses the chunk map should include the ALLOC_TABLE spec in their global sidemetadata specs.
The allocation state for a chunk in the chunk map. It includes whether each chunk is allocated or free, and the space the chunk belongs to.
Highest bit: 0 = free, 1 = allocated
Lower 4 bits: Space index (0-15) if the chunk is allocated.