Merge: Popcorn valid

Merge: Popcorn valid

## Quick and easy validation framework for Json inputs

Validators can be used in Popcorn apps to valid your json inputs before
data processing and persistence.

Here an example with a Book management app. We use an ObjectValidator to validate
the books passed to the API in the `POST /books` handler.

~~~nit
import popcorn
import serialization

# Serializable book representation.
class Book
	super Jsonable

	# Book ISBN
	var isbn: String

	# Book title
	var title: String

	# Book image (optional)
	var img: nullable String

	# Book price
	var price: Float
 end

 class BookValidator
	super ObjectValidator

	redef init do
		add new ISBNField("isbn")
		add new StringField("title", min_size=1, max_size=255)
		add new StringField("img", required=false)
		add new FloatField("price", min=0.0, max=999.0)
	end
 end

 class BookHandler
	super Handler

	# Insert a new Book
	redef fun post(req, res) do
		var validator = new BookValidator
		if not validator.validate(req.body) then
			res.json_error(validator.validation, 400)
			return
		end
		# TODO data persistence
	end
 end
 ~~~

Pull-Request: #2240
Reviewed-by: Jean Privat <jean@pryen.org>