Draft: Incremental JSON parser

Context

The old JSON parser wasn't incremental. It often parsed data sent over the network in packets. If the first packet didn't contain an entire value, the parser waited for more data, concatenated both packets and started over from the beginning.

This new parser is incremental, which means that it only needs to wait for more data to continue, without having to re-parse the first packet.

Manually testing the MR

There are both expect_test and pbt tests, that can be run with dune runtest.

The new parser lives in destruct_incremental.ml.