Assignment to \(@array)

Assignment to \(@array)

This is a slurpy lvalue that gobbles up all the rhs elements, which
are expected to be references.  So \(@a)=\(@b) makes @a share the
same elements as @b.

We implement this by pushing a null on to the stack as a special
marker that pp_aassign will recognise.

I decided to change the wording for the \local(@a)=... error
slightly, from what my to-do tests had.

Some of the other to-do tests were badly written and had to be
fixed up a bit.