Add Thread.join as an alias for Thread.finish

A lot of people from other programming environment backgrounds will
likely try Thread.join, and get very confused when it compiles and
runs but does not synchronize.  Also, the chances of wanting to
perform a string join with a thread is low

src/core/Thread.pm

@@ -45,6 +45,10 @@ my class Thread {
         self
     }
 
+    method join(Thread:D:) {
+        self.finish
+    }
+
     multi method Str(Thread:D:) {
         "Thread<$.id>($.name)"
     }