reverseSlot = slot => {
  const otherDir = slot[1] === '+' ? '-' : '+';
  const revslot = `${slot[0]}${otherDir}${slot.slice(2)}`;
  return revslot;
}
Reverse slot direction.

Reversed to prevent namespace collisions between slots we
allocate and the ones the other side allocates.  If we allocate
a slot, serialize it to the other side, and they send it back to
us, we need to reference just our own slot, not one from their
side.

@param {CapTPSlot} slot
@returns {CapTPSlot} slot with direction reversed