Add is_thunk to QAST::Block.
Add is_thunk to QAST::Block.

For now just as another attribute; there's now 3 of these that could
flatten into a single attribute of flag bits.

cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp,cygx/nqp