add DEFINE-MORE-FUN, use it for vararg arithmetic functions

add DEFINE-MORE-FUN, use it for vararg arithmetic functions

  More efficient than consing a rest-list -- even a stack-allocated one, and
  doesn't add extra DX cleanup frames to backtraces.

  Done this way instead of just using &MORE directly in lambda-lists in order
  to mangle the lambda-list into &REST shape for user-consumption.