shh_newFilter: update API to allow complex topics

[karalabe]

The idea is to allow specifying potentially nested topic lists to create complex filter criteria. At the highest level the topics argument is still a list, but each element may be one of:

• string: as used previously
• null: signalling a wildcard/match-all
• string array: to match any element within

A few samples:

shh.filter({}).watch(function(err, msg) {
    console.log("Empty catcher:", msg)
})
shh.filter({topics: ["singleton"]}).watch(function(err, msg) {
    console.log("Single catcher:", msg)
})
shh.filter({topics: ["a", "b"]}).watch(function(err, msg) {
    console.log("Multi catcher:", msg)
})
shh.filter({topics: [["1", "2"]]}).watch(function(err, msg) {
    console.log("Binary catcher:", msg)
})
shh.filter({topics: [null, "x"]}).watch(function(err, msg) {
    console.log("Wildcard catcher:", msg)
})

These currently produce these JSON requests, the last two of which seem wrong

[{"topics":[]}]
[{"topics":["0x73696e676c65746f6e"]}]
[{"topics":["0x61","0x62"]}]
[{"topics":["0x5b2231222c2232225d"]}]
[{"topics":["0x6e756c6c","0x78"]}]

[frozeman]

Thats currently not supported. @debris we would need to modifiy the topics parser to parse even two dimensions and transform values to hex (if not already)

Though it would affect eth filters as well, so we have to make sure that when using eth its flattened afterwards.

[frozeman]

@debris Thats how topics filtering in whisper is now implemented in go:

[A, B] = A && B
[A, [B, C]] = A && (B || C)
[NULL, A, B] = ANYTHING && A && B

null is wildcard

we should make sure its the same in cpp and add this to the docs.

[frozeman]

fixed in c8f4226