EIP-2481: Testcases + proposed change to GetBlockHeaders

[holiman]

This PR adds testcase for each of the messages that were redefined as part of eth/66.
Since at the time of creating this PR, there are no clients that have implemented these messages, it would be good if the rlp-encoding of the mssages could be properly defined in the specification, and possibly also that others can verify these rlp blobs.

This PR also proposes a change to the RLP-format of GetBlockHeaders.

Before this change:
EDIT: This is now the 'correct' way, unchanged from the original spec:

rlpdump -hex 0xca820457c682270f050580

[
  0457,
  [
    270f,
    05,
    05,
    "",
  ],
]

After this change:
EDIT: The example below is moot:

rlpdump -hex c982045782270f050580
[
  0457,
  270f,
  05,
  05,
  "",
]

[eip-automerger]

Hi! I'm a bot, and I wanted to automerge your PR, but couldn't because of the following issue(s):

• EIP 2481 requires approval from one of (christoph@ethereum.org)

[holiman]

• I removed the reference to les,
• Testcases regenerated
• Removed the example from the abstract (the actual spec is just a little bit further down, seems unnecessary to duplicate things into the abstract)

[cburgdorf]

Thanks for making these changes. LGTM 👍