Create an iterator for Jsmn::Object arrays

[ZmnSCPxj]

Currently, we iterate over JSON arrays by using a standard for-i-from-0-to-length loop. Then we index the JSON array by [i] syntax. Unfortunately, due to how JSMN structures are, the [i] lookup is O(n), so iteration is O(n^2).

We can instead use an iterator, which only requires a operator++ operation, which can be O(1) with the JSMN structures. This probably requires that array Jsmn::Object wrappers cache their end iterator.

[ZmnSCPxj]

Fixed. An iterator is now available, which should lead to faster processing of JSON results. Not all possible use-cases have been changed to use iterators, but the heavy hogs (mostly channel finders) have been updated appropriately.