[feature] allow specify "map" function for deserializing #24
Description
Imagine serialized data having some bug.
For user-defined types it is possible to apply fix by providing proper ocaml.type "unserializer".
For top-level messages it is possible to overshadow with custom module implementing "correction" logic (it would help to be able to rename generated ocaml module for such top-level messages).
Bug for extprot records used inside other extprot types it is not possible to "inject" some filter during unserialization.