Improve definition of log filter topic JSON type

[fjl]

There were two problems in the type definition of the log filter "topics" field:

• Topics must be encoded as bytes32 instead of uint256. Implementations reject hex topics with less than 32 bytes.
• Items of the topic list can be given as a single topic or as an array of multiple topics. In the latter case, the filter returns logs with any of the given topics.

Example: the filter below matches logs with

• any topic in the first position
• topic 0xd783efa4... in the second position
• topic 0x1ddcc4de... OR 0x77d14ee1... in the third position.

{
  "topics": [
    [],
    "0xd783efa4d392943503f28438ad5830b2d5964696ffc285f338585e9fe0a37a05",
    ["0x1dcc4de8dec75d7aab85b567b6ccd41ad312451b948a7413f0a142fd40d49347", "0x77d14e10470b5850332524f8cd6f69ad21f070ce92dca33ab2858300242ef2f1"]
  ]
}

[lightclient]

Thank you for this correction.

[fjl]

Please also check #129