Integrate taocpp::json

[ColinH]

This pull request

• uses tao::optional, which is an alias for std::optional when available, instead of boost::optional, and
• uses taocpp/json for JSON string escaping (in a more efficient way).

Note that tao::optional is a part of taocpp/json because JSON needs an optional in its API, and we include a copy of an experimental optional to keep compatibility with C++11.

[codecov]

Codecov Report

Merging #69 into master will increase coverage by 0.12%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
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+ Coverage   16.76%   16.89%   +0.12%     
==========================================
  Files         184      183       -1     
  Lines       31025    30793     -232     
==========================================
  Hits         5202     5202              
+ Misses      25823    25591     -232

[lambdafu]

Ok, thanks. Migrating away from boost for standard stuff is certainly desirable. I am not invested heavily in nlohmann/json at this point, but I am invested in pegtl and taocpp/json is powerful and fast, so I don't mind migrating. There is a performance comparison at https://github.com/miloyip/nativejson-benchmark, where taocpp/json comes out ahead of nlohmann in performance, although not in code size.