31      23      15      7      0
CAR: XXXXXXXXXXXXXXXXXXXXXXXENNTTTTAM
CDR: XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

M: marked by GC
A: atom
  1: atom
  0: pair
TTTT: atom type
  0000: Symbol		cdr: pointer to c-string
  0001: Char		cdr: unicode codepoint
  0010: Vector		car: num elements; cdr: pointer to first element
x 0011: Lambda		cdr: (params (expression ...) env)
  0100: Prim		cdr: pointer to c-function
  0101: Number		N: number type; E: exactness; cdr: number
  0110: String		car: length; cdr: pointer to c-string
  0111: Port		car: fd
  1000: Special
  1001: Free

Vector
  init: 	()

Special constants:
  empty		()
  true		#t
  false		#f

- block of 1024 objects
- growing array of pointers to blocks
- freelist