Introduce --read_checked_file option

[mtzguido]

This PR introduces an option to make F* read a checked file and print it out (the tc_module in it, actually). It does not involve any include path, or finding the corresponding file, or parsing: it just reads whatever file the user passes in and prints it to standard output.

$ ./bin/fstar.exe --read_checked_file ulib/.cache/prims.fst.checked
module Prims
Declarations: [
assume new
val attribute:Type0
assume
val cps:Prims.attribute
assume
val hasEq (_: Type) : Prims.GTot Type0
let eqtype = a: Type0{Prims.hasEq a}
assume new
val bool:Prims.eqtype
type empty =
type trivial = | T : Prims.trivial

assume new
val unit:Prims.eqtype
let squash p = x: Prims.unit{p}
let auto_squash p = Prims.squash p
private
let logical = Type0
[...]

It would be nicer if we improve the format of modul_to_string, since it currently displays everything twice. See this comment:

(*
 * AR: 07/19: exports is redundant, keeping it here until vale is fixed to not parse it
 *)
let modul_to_string (m:modul) =
  U.format3 "module %s\nDeclarations: [\n%s\n]\nExports: [\n%s\n]\n" (sli m.name)
                                                                     (List.map sigelt_to_string m.declarations |> String.concat "\n")
                                                                     (List.map sigelt_to_string m.declarations |> String.concat "\n")

[mtzguido]

I'm merging this since it could be useful. We can also consider printing other pieces of the checked file (deps, hashes, etc).