No more default constructors.

Fixes #296.

test/core/list/join.wren

@@ -15,6 +15,7 @@ IO.print([1, [2, [3], 4], 5].join(",")) // expect: 1,[2, [3], 4],5
 
 // Calls toString on elements.
 class Foo {
+  construct new() {}
   toString { "Foo.toString" }
 }
 

test/core/list/to_string.wren

@@ -9,6 +9,7 @@ IO.print([1, [2, [3], 4], 5]) // expect: [1, [2, [3], 4], 5]
 
 // Calls toString on elements.
 class Foo {
+  construct new() {}
   toString { "Foo.toString" }
 }