No more default constructors.

Fixes #296.

test/core/sequence/count.wren

@@ -1,4 +1,6 @@
 class TestSequence is Sequence {
+  construct new() {}
+
   iterate(iterator) {
     if (iterator == null) return 1
     if (iterator == 10) return false

test/core/sequence/is_empty.wren

@@ -2,6 +2,7 @@ IO.print([].isEmpty) // expect: true
 IO.print([1].isEmpty) // expect: false
 
 class InfiniteSequence is Sequence {
+  construct new() {}
   iterate(iterator) { true }
   iteratorValue(iterator) { iterator }
 }

test/core/sequence/map.wren

@@ -15,6 +15,8 @@ class FibIterator {
 }
 
 class Fib is Sequence {
+  construct new() {}
+
   iterate(iterator) {
     if (iterator == null) return FibIterator.new()
     iterator.iterate

test/core/sequence/no_constructor.wren

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

test/core/sequence/to_list.wren

@@ -1,4 +1,6 @@
 class TestSequence is Sequence {
+  construct new() {}
+
   iterate(iterator) {
     if (iterator == null) return 1
     if (iterator == 3) return false

test/core/sequence/where.wren

@@ -15,6 +15,8 @@ class FibIterator {
 }
 
 class Fib is Sequence {
+  construct new() {}
+
   iterate(iterator) {
     if (iterator == null) return FibIterator.new()
     iterator.iterate