Non-standard JSON Pointer `#` syntax and indexes with leading zeros

[jg-rp]

As mentioned in #84, when using our non-standard "current key" syntax, we are failing to check for leading zeros when resolving array indexes.

RFC 6901 defines array-index as

array-index = %x30 / ( %x31-39 *(%x30-39) )
              ; "0", or digits without a leading "0"

Our # syntax should follow the same rules regarding indexes and leading zeros.