Change how new operators are compiled.

Instead of an instruction that always instantiates an instance,
the "new" is compiled to a method call on the class, followed by a
method call on the (presumably) new instance.

The former lets built-in types like Fn do special stuff instead of
allocating an instance. In particular, it means:

  new Fn { ... }

can just return the block argument.

test/function/type.wren

@@ -1,4 +1,4 @@
-IO.print((fn 0) is Function)      // expect: true
-IO.print((fn 0) is Object)        // expect: true
-IO.print((fn 0) is String)        // expect: false
-IO.print((fn 0).type == Function) // expect: true
+IO.print((fn 0) is Fn)      // expect: true
+IO.print((fn 0) is Object)  // expect: true
+IO.print((fn 0) is String)  // expect: false
+IO.print((fn 0).type == Fn) // expect: true