Be proactive in parser

[wvxvw]

Right now, the parse generated while reading the Proto file needs a lot of manipulations before it is possible to generate descriptors from it. A lot of that work can actually happen in the parser, potentially speeding up the process. In particular:

1. Normalize names of inner messages.
2. Extract oneof fields into parent message.
3. Extract inner messages into a flat list.
4. Define proto_file_t struct which provides easy access to the entities in the Proto file, something along the lines of:

   typedef struct proto_file_t {
   char* package;
   list_t* imports;
   list_t* enums;
   list_t* messages;
   } proto_file_t;

[wvxvw]

Most of this is done, but now it seems like there's a lot of duplication between types defined in helpers.h and in descriptors.h, in particular factory_t and field_info_t. Need to revisit this.