RFC6902 support

[thsutton]

This is a quick sketch based on glancing at some examples.

• Produce RFC6902 patches
• Apply RFC6902 patches
• Extract examples from the RFC as tests
• Correct accounting for position advances due to changes.

[thsutton]

Will address #8 when complete.

[thsutton]

The remaining bug seems to be in the way the keys for arrays are accounted for as changes accumulate. Here's an example bad diff and the source and target documents:

[{"op":"replace","path":"/0/0","value":{}}   // OK
,{"op":"remove","path":"/0/1"}                  // OK
,{"op":"replace","path":"/0","value":[]}      // BAD should be /1
,{"op":"add","path":"/1/�\u0003","value":[0]} // BAD should be /2
,{"op":"replace","path":"/1/","value":[false]} // BAD should be /2
]

[[[null],[]],{"":[-1]},{"":{"":-1}}]

[[{}],[],{"":[false],"�\u0003":[0]}]