No more default constructors.

Fixes #296.

test/language/nonlocal/mutual_recursion.wren

@@ -1,8 +1,10 @@
 class Foo {
+  construct new() {}
   static bar { Bar.new() }
 }
 
 class Bar {
+  construct new() {}
   static foo { Foo.new() }
 }
 

test/language/nonlocal/use_in_method.wren

@@ -1,6 +1,8 @@
 var Global = "global"
 
 class Foo {
+  construct new() {}
+
   method {
     IO.print(Global)
   }

test/language/nonlocal/use_in_method_before_definition.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   method {
     IO.print(Global)
   }