Adding simdjson to PECL (faster json decoder)

Hi,

I'd like to add https://github.com/crazyxman/simdjson_php to PECL.

During a previous proposal to add simdjson to php-src, one of the major review comments
was that people wanted to have it available as a PECL first to test it.

- https://externals.io/message/112638
- https://github.com/php/php-src/pull/6551

https://github.com/crazyxman/simdjson_php offers around a 2x-3x speedup in decoding json (`simdjson_decode`), and 5x-10x speedup on validating json (`simdjson_validate`)
on platforms supporting https://en.wikipedia.org/wiki/Single_instruction,_multiple_data
depending on the data being decoded.

- Sample benchmark results are available at https://github.com/crazyxman/simdjson_php/tree/master/benchmark

Since then, I've
- Addressed my own review comments on the php-src PR and added tests and become a maintainer of simdjson_php
- Fixed Windows build issues and ZTS issues and set up tests of those platforms
- Added optimizations

Any objections to adding this as a PECL?

Thanks,
- Tyson