pointer tags -
  bits 0..1
  00 - integer
  01 - float
  10 - object
  11 - reserved

object header -
  bit 0: GC mark bit
  bit 1: weak reference bit
  bits 2..29 - object size
  bit 30: reserved
  bit 31: reserved

object format -
  0: object header
  4: pointer to map
  
role entry format - 
  0: role position mask
  4: dispatch position mask
  8: pointer to method

slot entry format -
  0: pointer to slot name
  4: slot offset
  8: immutable slot value
  12: pointer to array of role entries

map format -
  0: object header
  4: pointer to representative object
  8: number of delegation slots
  12: number of data slots
  16: pointer to array of method dependencies
  20: oldest generation of weak references 
  24: dispatch id, 64 bits
  32: visited position mask
  36: slot entry 0
  36+16*N: slot entry N
  - slot entries are hashed, power-of-2 hash table size

method format additions -
  0: dispatch id, 64 bits
  8: visited position mask