Add "code_of_method" and "declares_method" methods

to NQPClassHOW.

- declares_method returns 1 if there's a method by that name in
  the method table (effectively wrapping an nqp::existskey)
- code_of_method returns the code object of the method by the given
  name, or nqp::null if there's no method by that name (effectively
  wrapping an nqp::atkey.

This is mostly to provide compatibility with methods by the same
name in Raku's ClassHOW, where the check is a little more involved
thanks to the existence of submethods.  By abstracting the lookup
it will now also be possible to remove a lot of checks for submethod
tables, as they can now be abstracted in methods with these names.

src/how/NQPClassHOW.nqp

@@ -808,6 +808,13 @@ knowhow NQPClassHOW {
     method method_table($XXX?)        { $!methods      }
     method tweaks($XXX?)              { $!tweaks       }
 
+    method declares_method($XXX, $name) {
+        nqp::existskey($!methods, $name)
+    }
+    method code_of_method($XXX, $name) {
+        nqp::atkey($!methods, $name)
+    }
+
     method methods($XXX?, :$local = 0, :$all) {
         if $local {
             nqp::clone($!method_order)