Inspect the expression instead of parsing its representation

[gpetiot]

Based on top of #10, and also the tests fixed in #9

The idea is to re-use the expressions (already parsed by the ocaml compiler) instead of re-parsing its string representation. It works because the config language is a subset of ocaml expressions. And this allows to lift some constraints to use more of the ocaml syntax, e.g. not enforcing parentheses if there is no ambiguity.

[KFoxder]

@gpetiot

1. This makes a ton of sense to me (and I don't know that much about OCaml) but I did feel like I was re-building parts of parsing OCaml syntax. Much simpler, cleaner to read too 👍
2. Should I close Fix: Make the equals operator work as expected. #9 and Fix: Allow the not token to be used. #7 and work off of this for future issues?