Hi Stefan,

The attached patch (inspired by your byte-code-function qpattern patch)
seems to add vector QPattern to pcase. Could you review it and give me
any comments? Thanks.

I haven't wanted vector qpattern badly enough until just now having put
up with many many:

     (and (pred vectorp) io
          (let `(io_request ,from ,replyas ,request)
            (cl-coerce io 'list)))

which is both ugly and inefficient.

Thanks,
Leo