Merge: lib/popcorn: introduce RepoObject in pop_repos

### Base serializable entity that can go into a JsonRepository

 Provide boiler plate implementation of all object serializable to json.

`id` is used as a primary key for `find_by_id`.

 Subclassing RepoObject makes it easy to create a serializable class:
 ~~~nit
 import popcorn::pop_repos

 class Album
	super RepoObject
	serialize

	var title: String
	var price: Float
 end
 ~~~

 Do not forget the `serialize` annotation else the fields will not be serialized.

 It is also possible to redefine the `id` primary key to use your own:
 ~~~nit
 import popcorn::pop_repos

 class Order
	super RepoObject
	serialize

	redef var id = "order-{get_time}"

	# ...

 end
 ~~~

Signed-off-by: Alexandre Terrasa <alexandre@moz-code.org>

Pull-Request: #2265
Reviewed-by: Jean Privat <jean@pryen.org>