No more default constructors.

Fixes #296.

test/language/super/call_different_arity.wren

@@ -5,6 +5,8 @@ class Base {
 }
 
 class Derived is Base {
+  construct new() {}
+
   foo(a) {
     IO.print("Derived.bar(a)")
     super

test/language/super/call_other_method.wren

@@ -5,6 +5,8 @@ class Base {
 }
 
 class Derived is Base {
+  construct new() {}
+
   bar {
     IO.print("Derived.bar")
     super.foo

test/language/super/call_same_method.wren

@@ -5,6 +5,8 @@ class Base {
 }
 
 class Derived is Base {
+  construct new() {}
+
   foo {
     IO.print("Derived.foo")
     super.foo

test/language/super/closure.wren

@@ -3,6 +3,7 @@ class Base {
 }
 
 class Derived is Base {
+  construct new() {}
   getClosure { Fn.new { super.toString } }
   toString { "Derived" }
 }

test/language/super/implicit_name.wren

@@ -5,6 +5,8 @@ class Base {
 }
 
 class Derived is Base {
+  construct new() {}
+
   foo {
     IO.print("Derived.foo")
     super

test/language/super/indirectly_inherited.wren

@@ -7,6 +7,8 @@ class A {
 class B is A {}
 
 class C is B {
+  construct new() {}
+
   foo {
     IO.print("C.foo")
     super.foo

test/language/super/no_superclass_method.wren

@@ -1,6 +1,7 @@
 class Base {}
 
 class Derived is Base {
+  construct new() {}
   foo { super.doesNotExist(1) } // expect runtime error: Base does not implement 'doesNotExist(_)'.
 }
 

test/language/super/super_in_closure_in_inherited_method.wren

@@ -6,6 +6,8 @@ class A {
   toString { "A.toString" }
 }
 
-class B is A {}
+class B is A {
+  construct new() {}
+}
 
 IO.print(B.new().callSuperToString) // expect: instance of B

test/language/super/super_in_inherited_method.wren

@@ -4,6 +4,8 @@ class A {
   toString { "A.toString" }
 }
 
-class B is A {}
+class B is A {
+  construct new() {}
+}
 
 IO.print(B.new().callSuperToString) // expect: instance of B