Make constructors just methods.

* Eliminate "new" reserved word.
* Allow "this" before a method definition to define a constructor.
* Only create a default constructor for classes that don't define one.

test/core/sequence/where.wren

@@ -1,6 +1,6 @@
 // Infinite iterator demonstrating that Sequence.where is not eager
 class FibIterator {
-  new {
+  this new() {
     _current = 0
     _next = 1
   }
@@ -16,7 +16,7 @@ class FibIterator {
 
 class Fib is Sequence {
   iterate(iterator) {
-    if (iterator == null) return new FibIterator
+    if (iterator == null) return FibIterator.new()
     iterator.iterate
     return iterator
   }
@@ -24,7 +24,7 @@ class Fib is Sequence {
   iteratorValue(iterator) { iterator.value }
 }
 
-var largeFibs = (new Fib).where {|fib| fib > 100 }
+var largeFibs = Fib.new().where {|fib| fib > 100 }
 var iterator = null
 
 IO.print(largeFibs is Sequence) // expect: true