Fibonaccing CID lengths

[Dmitri Tikhonov <[e-mail]>]

Idea: we can keep the one-byte SCIDL/DCIDL field but map the 4-bit
parts to a larger range of values.  For example, we could map the
16 values to a range of Fibonacci numbers:

    0   ->  0
    1   ->  1
    2   ->  2
    3   ->  3
    4   ->  5
    5   ->  8
    6   ->  13
    7   ->  21
    8   ->  34
    9   ->  55
    10  ->  89
    11  ->  144
    12  ->  233
    13  ->  377
    14  ->  610
    15  ->  987

This particular mapping has several profitable properties:

    - It covers the whole range of values from zero to the
      practical maximum imposed by the protocol (about 1K).

    - There is a good selection of small values.

    - 8 is one of the values, convenient for storage in
      uint64_t.

    - Each step up is not too large (double or more) or too
      small (less than 50% increase).

    - The mapping can be easily generated: its name says how.

  - Dmitri.