No more default constructors.

Fixes #296.

test/language/inheritance/inherit_fields.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   foo(a, b) {
     _field1 = a
     _field2 = b
@@ -11,6 +13,8 @@ class Foo {
 }
 
 class Bar is Foo {
+  construct new() {}
+
   bar(a, b) {
     _field1 = a
     _field2 = b

test/language/inheritance/inherit_methods.wren

@@ -6,6 +6,7 @@ class Foo {
 }
 
 class Bar is Foo {
+  construct new() {}
   methodOnBar { IO.print("bar") }
   method(a, b) { IO.print("bar") }
   method(a, b, c, d) { IO.print("bar") }

test/language/inheritance/is.wren

@@ -1,6 +1,12 @@
-class A {}
-class B is A {}
-class C is B {}
+class A {
+  construct new() {}
+}
+class B is A {
+  construct new() {}
+}
+class C is B {
+  construct new() {}
+}
 var a = A.new()
 var b = B.new()
 var c = C.new()