No more default constructors.

Fixes #296.

test/language/implicit_receiver/inherited_methods.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   getter {
     IO.print("getter")
   }
@@ -13,6 +15,8 @@ class Foo {
 }
 
 class Bar is Foo {
+  construct new() {}
+
   test {
     getter            // expect: getter
     setter = "value"  // expect: setter

test/language/implicit_receiver/instance_methods.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   getter {
     IO.print("getter")
   }

test/language/implicit_receiver/locals_shadow_getter.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   bar { "getter" }
 
   test {

test/language/implicit_receiver/locals_shadow_setter.wren

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

test/language/implicit_receiver/nested_class.wren

@@ -1,4 +1,6 @@
 class Outer {
+  construct new() {}
+
   getter {
     IO.print("outer getter")
   }
@@ -17,6 +19,8 @@ class Outer {
     method("arg")     // expect: outer method
 
     class Inner {
+      construct new() {}
+
       getter {
         IO.print("inner getter")
       }