Merge: Intro of nitrestful a RESTful API generator

This tool generate a Nit module which implements `Action::answer` to redirect request to a static Nit method. It checks the presence of args, their types, deserialize objects as needed and calls the target method.

Missing arguments or arguments with errors (wrong type, failed deserialization, etc.) are replaced by `null` when the corresponding parameter is `nullable`. If the parameter is non-nullable, or if there is any other error, the generated code calls `super` from `answer` for the user code to handle exceptions and errors.

With the `restful` annotation we can write a normal method with static types such as: (from the example)
~~~
# User code
class MyAction
	super RestfulAction

	# Method answering requests like `foo?s=some_string&i=42&b=true`
	fun foo(s: String, i: Int, b: Bool): HttpResponse is restful do
		var resp = new HttpResponse(200)
		resp.body = "foo {s} {i} {b}"
		return resp
	end
...
~~~

And nitrestful will generate for use the wrapper extracting the args from the request and call the method `foo`:
~~~
# Generated code by `nitrestful`
redef class MyAction
	redef fun answer(request, truncated_uri)
	do
		var verbs = truncated_uri.split("/")
		if verbs.not_empty and verbs.first.is_empty then verbs.shift
		if verbs.length != 1 then return super
		var verb = verbs.first

		if verb == "foo" then
			var in_s = request.string_arg("s")
			var out_s = in_s

			var in_i = request.string_arg("i")
			var out_i = deserialize_arg(in_i)

			var in_b = request.string_arg("b")
			var out_b = deserialize_arg(in_b)

			if not out_s isa String or not out_i isa Int or not out_b isa Bool then
				return super
			end
			return foo(out_s, out_i, out_b)
		end
		return super
	end
end
~~~

This is an early version of this tool. More work is needed to test different types, different usages, error management, and improve the docs and examples.

close #1852

Pull-Request: #1863
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <romain.chanoir@viacesi.fr>
Reviewed-by: Jean-Philippe Caissy <jpcaissy@piji.ca>