Major reorg

[lemire]

Eventually, we want to merge this code into simdjson, and this PR is a step in this direction. Furthermore, we want to make it easier for newcomers to contribute so we want to just outline the proper code and leave what is less essential to the side, where only interested reader might go.

This PR keeps only the essential files in include/simdjson/json_builder. The namespaces are changed. We also fallback on the simdjson convention where header files end with .h. I am aware that in C++, some people prefer .hpp, but that's not the current convention in simdjson. Elsewhere, .hpp headers are changed.

The PR moves the parser to the benchmarking directory. Much is moved there. For example, the dependencies are moved there so that if you want to just build the tests or the examples, you do not need to build the dependencies.

The CMake setup is now more standard in the sense that there is one project, and there are subdirectories.

The PR removes the small JSON parser from the examples and uses simdjson instead. This was done manually, but should be done automagically as per #29

The universal_formatter is kept in include/simdjson/json_builder but remains unused. We need to test it out (see #6). If anyone wants to play with that, it would be great.

cc @CarlosEduR

[FranciscoThiesen]

Thanks for taking the time to reorganize it. I am excited to be part of adding this to simdjson (:

[CarlosEduR]

Nice work @lemire!