topology: allow peering links between core-ASes

[matzf]

Currently, the topology loader forbids core ASes from having peering links (i.e. links with "link_to": "peer"). The validation check is here:

scion/private/topology/topology.go

Lines 544 to 550 in 4a8bc98

	if isCore {
	switch i.LinkType {
	case Core, Child:
	default:
	return serrors.New("Illegal link type for core AS",
	"type", i.LinkType, "br", brName)
	}

.
As far as I'm aware, there is no technical reason why core ASes cannot have peering links.
Core ASes should be able to have peering links to any other AS, i.e. to both core or non-core ASes in both the same or different ISDs.