No more default constructors.

Fixes #296.

test/core/object/no_constructor.wren

@@ -0,0 +1 @@
+Object.new() // expect runtime error: Object metaclass does not implement 'new()'.

test/core/object/not.wren

@@ -1,2 +1,4 @@
-class Foo {}
+class Foo {
+  construct new() {}
+}
 IO.print(!Foo.new()) // expect: false

test/core/object/same.wren

@@ -24,7 +24,9 @@ IO.print(Object.same(Bool, Num)) // expect: false
 IO.print(Object.same(Bool, Bool)) // expect: true
 
 // Other types compare by identity.
-class Foo {}
+class Foo {
+  construct new() {}
+}
 
 var foo = Foo.new()
 IO.print(Object.same(foo, foo)) // expect: true
@@ -32,6 +34,7 @@ IO.print(Object.same(foo, Foo.new())) // expect: false
 
 // Ignores == operators.
 class Bar {
+  construct new() {}
   ==(other) { true }
 }
 

test/core/object/to_string.wren

@@ -1,2 +1,4 @@
-class Foo {}
+class Foo {
+  construct new() {}
+}
 IO.print(Foo.new().toString == "instance of Foo") // expect: true