Feature: Do not use nlohmann/json, use a faster to compile and use JSON

[davidmoreno]

I have the theory that slow compilation times is because of use of this library. It is very nice and easy to use, but slow to compile

And compilation times are exploding.

Find an alternative, and replace.

Possible alternatives:

• rapidjson - https://github.com/Tencent/rapidjson
• jsoncpp - https://github.com/open-source-parsers/jsoncpp

Alternative is to move all functions that really use json to other compilation units. But as every midipeer_t has one, its can get a bit messy.