Merge: Advice on useless repeated types

With this PR, nitpick and other tools are able to suggest to remove useless types in signature redefinition with the option `-W`.

Example:

~~~nit
class A
	fun foo(a: Bool) do end
end

class B
	super A

	redef fun foo(a: Bool) do end
end
~~~

~~~
$ nitc test.nit -W
test.nit:8,19--22: Warning: useless type repetition on parameter `a` for redefined method `foo` (useless-signature)
		redef fun foo(a: Bool) do end
		                 ^
Errors: 0. Warnings: 1.
~~~

Pull-Request: #1373
Reviewed-by: Lucas Bajolet <r4pass@hotmail.com>
Reviewed-by: Jean Privat <jean@pryen.org>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>