No more default constructors.

Fixes #296.

test/language/closure/close_over_method_parameter.wren

@@ -1,6 +1,8 @@
 var F = null
 
 class Foo {
+  construct new() {}
+
   method(param) {
     F = Fn.new {
       IO.print(param)

test/language/closure/closed_closure_in_method.wren

@@ -4,6 +4,8 @@ var foo = null
 {
   var local = "local"
   class Foo {
+    construct new() {}
+
     method {
       IO.print(local)
     }

test/language/closure/open_closure_in_method.wren

@@ -2,6 +2,8 @@
 {
   var local = "local"
   class Foo {
+    construct new() {}
+
     method {
       IO.print(local)
     }