Rewrite the internal structure handling Parsifal constructions

[pictyeye]

To be able to reason about the constructions before issuing let rec statements, there is a need to rewrite the internal structure describing Parsifal constructions.

A priori, such a structure should contain the following fields:

• the PType name
• its dependecies
• the OCaml type expression
• optional non-recursive definitions (enrich references for unions, int_of_t and other enum-specific functions)
• the parse_t definition
• the dump_t definition
• the value_of_t definition