Add and use type definitions for structures and enumerations

Add and use type definitions for structures and enumerations

Using the `_t` suffix to avoid changing the current API because `enum
argpar_item_type` has the same name as the argpar_item_type() function.

Using those new type definitions everywhere to make sure they work.

Signed-off-by: Philippe Proulx <eeppeliteloop@gmail.com>
Change-Id: I87bcd2abd40ff91779b9c82b4b9b8a7afb535fb3