Add Boost.Beast experimental streaming parser

[mloskot]

The parser implementation is available from Boost.Beast's demo project here
https://github.com/vinniefalco/BeastLounge/tree/develop/include/boost/beast/_experimental/json

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UPDATE: Current code lives in https://github.com/vinniefalco/json

/cc @vinniefalco

[vinniefalco]

Yeah, I would love to see a benchmark BUT full disclosure, I have spent exactly zero time optimizing this code (LOL)

[mloskot]

@vinniefalco I wouldn't worry about it. The benchmark compares what is available 'on the market' as it is.
AFAIK, there has been just first round of implementation of the Ciere's Spirit-based parser, so it has not been optimised either.
Still, it is valuable to have it compared, for both, its developers and its users.
I believe, this benchmark is for purely practical purposes and not about building any halls of (sh|f)ame.

[vinniefalco]

The streaming parser is separate from the non-streaming parser now. They have different performance characteristics.