A few changes needed by qast_restructure in NQP.

Only one known regressions remains, which is related to compile-time
inlining of multis.
teach clone_qast (and thus node_walker) about QAST::Children
Toss clone_qast sub; there's a method for that.
Fix is_clearly_returnless for empty block case.

Not sure how this ever really worked out in the past; perhaps a
QAST tree mis-construction that's now impossible made it appear
to work.
Bump NQP_REVISION to for restructured QAST nodes.
Revert "Merge branch 'qast_refactor' into nom"

This reverts commit 54cde6bfd1dd23bd7998334b602183e1ee134b31, reversing
changes made to 356d57aabbc54eb2d6030236ebdff02ebe992335.
Merge branch 'qast_restructure' into nom