7A = 0111 1010 = 3b lookback 0x6 bytes
36 = 0011 0110 = 2b lookback 0xa bytes

1 = literal
01 = 1b lookback:
  - high 2 bits are count (add 2)
  - OR low 6 bits with FFC0 and treat as 16-bit signed offset from current pos
001 = 2b lookback:
  - high 4 bits are count?
    - if zero, alt handling:
      - read another byte
      - if nonzero, add 2 to get real length
      - otherwise, ???
    - otherwise, add 2 to get real count
  - low 12 bits are offset: OR with F000 and treat as signed
