change JsonDecoder to handle partial array updates

[megahirt]

Description

This is a change to JsonDecoder to be able to handle partial updates. This is a work in progress. The purpose of this PR work is to prove out an alternate strategy for handling delta updates as @rmunn implemented in #1204

I also added some unit tests to demonstrate partial updates to arrays.

Type of Change

• New feature (non-breaking change which adds functionality)

Checklist:

• I have performed a self-review of my own code
• I have reviewed the title/description of this PR which will be used as the squashed PR commit message
• I have commented my code, particularly in hard-to-understand areas
• I have added tests that prove my fix is effective or that my feature works

[megahirt]

@rmunn I realize now that the approach you have taken is reasonable given the complexities of doing array delta updates. There is no way around having a separate "delta update" on the back-end as far as I can see. I have spent more time in the JsonDecoder and added a $isDeltaUpdate flag to the decode() method, which defaults to false so that existing functionality is preserved. I have one unit test that takes advantage of the $isDeltaUpdate flag set to true to demonstrate a changed item on just one element of the array, but this is just the start.

The basic elements and strategy of your changes in #1204 seem necessary to me, namely:

• the client must determine if changes can be sent as a deltaUpdate or a regular update (whole structure). If an array has been rearranged or had deletions, then this must be a regular update
• the client must tell the server whether to do a delta update or a regular update. The data itself doesn't specify the type of update. I think you setup a separate RPC for the delta update. It could be done like this or, the $isDeltaUpdate flag could be passed all the way from the client into the update call.

This is still a draft PR, but tests are passing now.

TODO:

• implement similar deltaUpdate feature for MapOf type
• write tests for MapOf type
• write more tests for ArrayOf type
• write LexEntryUpdate tests to test entry update scenarios

[megahirt]

Abandoning this PR as Robin's work on delta updates supersedes this experimental work.