No more default constructors.

Fixes #296.

test/language/setter/instance.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   bar=(value) {
     IO.print(value)
   }

test/language/setter/result.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   bar=(value) { "result" }
 }
 

test/language/setter/same_name_as_method.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   bar=(value) { IO.print("set") }
   bar { IO.print("get") }
 }