No more default constructors.

Fixes #296.

test/language/variable/local_in_middle_of_block.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   bar {
     var a = "a"
     IO.print(a) // expect: a

test/language/variable/outside_method.wren

@@ -1,6 +1,8 @@
 var foo = "variable"
 
 class Foo {
+  construct new() {}
+
   foo { "method" }
 
   method {