Use concat overloading for "foo$_->$*"

This is the only discrepancy between $$_ and $_->$* that I know about.

To get ->@... interpolation to work, we have to emit a special
POSTJOIN token, which has just the right precedence to get it to apply
to the right amount of code before it, which perly.y then turns into a
regular join(...).

->$* and ->$#* were also going through that same code path, though it
turns out that simply omitting the POSTJOIN token for these dollar
tokens Just Works.  (I thought the fix would be more complicated.)

Now $_->$* within quotes becomes a direct argument to the concat ope-
rator, instead of being wrapped in a stringify(...) (what join(...)
optimises to with a single-item list).