Add JSON backend

[wxrdnx]

I've implemented a simple JSON backend that dumps the internal Sail AST into a JSON file.
The goal is to convert the AST into a cross-language representation that it can be used by languages other than Ocaml.

[Alasdair]

Rather than using string interpolation like

let rec json_of_value = function
  | V_vector values -> sprintf "{\"V_vector\":%s}" (json_of_list json_of_value values)

The better way to do this in OCaml is to use polymorphic variants, which is how we represent JSON elsewhere, e.g the above would be

let rec json_of_value = function
  | V_vector values -> `Assoc [("V_vector", json_of_list json_of_value values)]

The type we use is the Yojson Safe variant https://ocaml.org/p/yojson/3.0.0/doc/yojson/Yojson/Safe/index.html. With polymorphic variants you don't actually need to import the library, as they are structurally typed, but you can for type annotations.