Merge: Check operator

Previously, no checking was done on the signature of operators.
While this is not an issue for the model nor the tools this could yield to not POLA error messages when the operator is used. eg.

~~~nit
class A
   fun +(other: A) do print "hello"
end
var a = new A
var b = a + a # Error expected expression
a + a # Error unexpected operator +
# no way do invoke `+` in fact.
~~~

With the PR, we have

~~~
Error: mandatory return type for `+`.
fun +(other: A) do print "hello"
    ^
~~~

The following errors are added by the PR:

* mandatory return
* not enough parameters
* too much parameters
* illegal variadic parameter

Pull-Request: #1269
Reviewed-by: Alexandre Terrasa <alexandre@moz-code.org>
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Romain Chanoir <chanoir.romain@courrier.uqam.ca>