Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

test/language/while/closure_in_body.wren

@@ -3,7 +3,7 @@ var list = []
 var i = 1
 while (i < 4) {
   var j = i + 1
-  list.add(new Fn { IO.print(j) })
+  list.add(Fn.new { IO.print(j) })
   i = i + 1
 }
 

test/language/while/return_closure.wren

@@ -1,7 +1,7 @@
-var f = new Fn {
+var f = Fn.new {
   while (true) {
     var i = "i"
-    return new Fn { IO.print(i) }
+    return Fn.new { IO.print(i) }
   }
 }
 

test/language/while/return_inside.wren

@@ -1,4 +1,4 @@
-var f = new Fn {
+var f = Fn.new {
   while (true) {
     var i = "i"
     return i