Move JSON to mllib

Move the simple OCaml JSON writer to mllib, so that can be enhanced and
used also outside v2v.
mllib: improve JSON: more types, indented output style

Improve the homebrew JSON writer:
- add more types (including also nested dictionaries and lists)
- format in a compact way (single line), or indented (multilines)
builder: use the JSON module

Switch to the JSON OCaml module for JSON output.

The resulting output is the same, except from an indentation level more
within lists.