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.

benchmark/delta_blue.wren

@@ -428,7 +428,7 @@ class Variable {
 
   // Removes all traces of c from this variable.
   removeConstraint(constraint) {
-    _constraints = _constraints.where(fn (c) c != constraint)
+    _constraints = _constraints.where { |c| c != constraint }
     if (_determinedBy == constraint) _determinedBy = null
   }
 }