Fix chicken/egg problem with Lock::Async::Holder

Trying to call .new at compile time on a class that refers to an outer
class that has not been composed yet, explodes at least during setting
compilation (without much useful feedback).

Since the .new was without parameters, a simple nqp::create is enough.

This allows setting compilation to complete with auto-generated BUILDALL
installed.  But "make install" still fails  :-(

src/core/Lock/Async.pm

@@ -49,7 +49,7 @@ my class Lock::Async {
 
     # Base states for Holder
     my constant NO_HOLDER = Holder;
-    my constant SINGLE_HOLDER = Holder.new;
+    my constant SINGLE_HOLDER = nqp::create(Holder);
 
     # The current holder record, with waiters queue, of the lock.
     has Holder $!holder = Holder;