Add MVMNull REPR, instance->VMNull.
Add static inline func for doing a VM-null check.
Make nqp::null and nqp::isnull VM-null aware.
Teach serialization about VM nulls.
Teach P6opaque about VMNull; eliminate ass_null.
Teach ifnonnull about VMNull.
Update continuations code for VMNull.
Update invocation handling for VMNull.
Factor VMNull into null HLL mapping.
Make array and hash use VMNulls; fix fallout.

With this, NQP still builds/passes tests.
Use MVM_is_null in gethllsym.
Use MVM_is_null in sp_findmeth.
A couple more placs MVM_is_null is needed.

With this, Rakudo builds and sanity-tests again.
Do VM null check in event loop.