Set up a BOOTArray type.
Make nqp::hlllist return the BOOTArray type.

In the best of all worlds, it'll return types from the HLL config. But for
that to work, I'll have to implement the HLL config first.
Fix a bug in the new nqp::list and nqp::list_b.
Implement VTABLE_elements in sixmodelobject.pmc and elems in VMArray.
Fix a bug in VMArray's unshift.
Refactor set_boolification_spec logic out of nqp.ops.
Implement HAS_ELEMS boolification mode.
First implementation of VMIter REPR.
Refactor sixmodel initialization to store objects in a global context hash.