a very very WIP first cut of peeking via MSC2753.

doesn't yet compile or work.
needs to actually add the peeking block into the sync response.
checking in now before it gets any bigger, and to gather any initial feedback on the vague shape of it.

syncapi/types/types.go

@@ -507,3 +507,27 @@ type SendToDeviceEvent struct {
 	DeviceID    string
 	SentByToken *StreamingToken
 }
+
+// For tracking peeking devices
+
+type PeekingDevice struct {
+	ID     string
+	UserID string
+}
+
+type PeekingDeviceSet map[PeekingDevice]bool
+
+func (s PeekingDeviceSet) add(d PeekingDevice) {
+	s[d] = true
+}
+
+func (s PeekingDeviceSet) remove(d PeekingDevice) {
+	delete(s, d)
+}
+
+func (s PeekingDeviceSet) values() (vals []PeekingDevice) {
+	for d := range s {
+		vals = append(vals, d)
+	}
+	return
+}