Replace our own JSON library, lexer and parser with Yojson

[sim642]

We already integrate Yojson, so we might as well use it for everything. Currently the config system uses our own JSON structures and we have our own JSON lexer and parser.

There's also JsonSchema that checks this custom JSON structure, but if we want to keep that, I guess it could still be adapted to just look at a Yojson structure instead.

[sim642]

What makes this much more difficult is that Yojson uses a nice and pure data type. But our JSON data type is mutable:

analyzer/src/util/json.ml

Lines 11 to 18 in 45e40c0

	(** Json data structure *)
	type jvalue =
	| String of string
	| Number of num
	| Object of jvalue ref Object.t ref
	| Array of jvalue ref list ref
	| True | False
	| Null

So when setting options, we just mutate some ref inside it and that's it. To use Yojson, one would have to recursively reconstruct the modified structure around the change.