Expose some MOP primitives for Perl 6 programmers.

Now the 6model API has been stable for a while, we should expose some
key aspects of it. So far, short of using nqp::ops, it has not been
possible for a Perl 6 meta-programmer to totally create their own
kinds of type from scratch; customizing existing ones has been the
only way. Custom publishing of method caches has also not been on
offer, and some modules could certainly make use of it. Now it will
be possible.