Compile the protocol's environment with coq-of-ocaml

Reopening of !153 (merged)

With this MR, we allow the compilation of the protocol environment in Coq using coq-of-ocaml. Compared to the previous MR, we do minimal changes so that we only touch the environment in this MR.

The changes are:

• name the anonymous signatures (to translate them to records with a name);
• name the polymorphic variant key_or_dir;
• name the polymorphic variant tag_size;
• replace a use of with group in a module signature by its corresponding with type;
• tag the modules of the environment as @@coq_plain_module, so that they are not transformed to records.