cmd/rlpdump: add support for text to rlp

[holiman]

This PR adds support for the rlpdump tool to go from text format to RLP. This is often a bit cumbersome: modifying RLP means that the lengths needs to be recalculated. Right now, I often create some golang testcase which defines some struct and then call rlp.EncodeToBytes on it, which is a bit PITA.

Using a moderately complext struct:

[user@work rlpdump]$ ./rlpdump -hex 0xd8c1c0d5d4cccb84746573742a2a808213378667617a6f6e6b 
[
  [
    [],
  ],
  [
    [
      [
        [
          "test",
          "*",
          "*",
          "",
          1337,
        ],
      ],
      "gazonk",
    ],
  ],
]

Examples:

[user@work rlpdump]$ ./rlpdump -hex 0xd8c1c0d5d4cccb84746573742a2a808213378667617a6f6e6b | ./rlpdump -reverse 
0xd8c1c0d5d4cccb84746573742a2a808213378667617a6f6e6b
[user@work rlpdump]$ ./rlpdump -hex 0xd8c1c0d5d4cccb84746573742a2a808213378667617a6f6e6b > foo.test && ./rlpdump -reverse foo.test
0xd8c1c0d5d4cccb84746573742a2a808213378667617a6f6e6b

It doesn't handle 'bad input' well, and can panic, but that's not really important for this tool, IMO.

[holiman]

cc @marioevz you might find this useful too, when messing with e.g. transactions.

[holiman]

@MariusVanDerWijden PTAL?

[MariusVanDerWijden]

SGTM