Grammar rules as documentation on the AST module

[W95Psp]

Following a discussion with @karthikbhargavan, @cmester0 and I have been experimenting with putting OCaml comments directly onto the AST module, documenting the grammar rules covered by the internal AST of hax.

However, we believe that's not the best way to do things.

@cmester0 had a great idea: use the rust printer of hax to generate the grammar.

The idea is the following:

• we define two top-level values for each type: enumerate and default
• enumerate for type t is of type t set, default for type t is of type t
• enumerate for each t scalar will be a default value (say 0 for ints, "" for strings, etc.)
• enumerate of t list would be all possible combinations of the values enumerate<t> in list of length from zero to say 3
• enumerate of t option would be the set None and Some of all the values in enumerate for type t
• for every type of the AST, we derive via PPX default and enumerate. We need to make sure those enumerate are finite somehow (we can have a seen list of values, or do something statically, let's see)
• use the rust printer to generate rules

Status: let's not merge this