Improve generic type parsing and referencing

[metoule]

The arity of the generic type is now considered when trying to get the reference type (e.g. Tuple<,,> is now looked up as Tuple`3 during parsing).

That means it's possible to write:

var target = new Interpreter();
target.Reference(typeof(Tuple<>));
target.Reference(typeof(Tuple<,>));
target.Reference(typeof(Tuple<,,>));
Assert.AreEqual(1, target.Eval("new Tuple<int>(1).Item1"));
Assert.AreEqual("My str item", target.Eval("new Tuple<int, string>(5, \"My str item\").Item2"));
Assert.AreEqual(3, target.Eval("new Tuple<int, int, int>(1, 2, 3).Item3"));

The old way is still working, but in that case you need to add the arity yourself:

var target = new Interpreter();
target.Reference(typeof(Tuple<>), "Toto`1");
target.Reference(typeof(Tuple<,>), "Toto`2");
target.Reference(typeof(Tuple<,,>), "Toto`3");
Assert.AreEqual(1, target.Eval("new Toto<int>(1).Item1"));
Assert.AreEqual("My str item", target.Eval("new Toto<int, string>(5, \"My str item\").Item2"));
Assert.AreEqual(3, target.Eval("new Toto<int, int, int>(1, 2, 3).Item3"));

I took this opportunity to also centralize the way types are parsed, so that it's consistent everywhere.

Fixes #174

[davideicardi]

Great! Just one question on the resx ...