No more default constructors.

Fixes #296.

test/language/field/default_to_null.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   write { IO.print(_field) }
 }
 

test/language/field/multiple.wren

@@ -1,4 +1,6 @@
 class Foo {
+  construct new() {}
+
   set(a, b, c, d, e) {
     _a = a
     _b = b

test/language/field/nested_class.wren

@@ -1,9 +1,13 @@
 class Outer {
+  construct new() {}
+
   method {
     _field = "outer"
     IO.print(_field) // expect: outer
 
     class Inner {
+      construct new() {}
+
       method {
         _field = "inner"
         IO.print(_field) // expect: inner

test/language/field/use_before_set.wren

@@ -1,4 +1,5 @@
 class Foo {
+  construct new() {}
   write { IO.print(_field) } // Compile a use of the field...
   init { _field = "value" }  // ...before an assignment to it.
 }