Add client core, handshake pending frames, and in-memory Conn tests

message.go

@@ -0,0 +1,25 @@
+package ownwire_sdk
+
+type EventKind uint8
+
+const (
+	EventOpened EventKind = iota + 1
+	EventMessage
+	EventError
+	EventClosed
+)
+
+type Event struct {
+	Kind    EventKind
+	Message Message
+	Err     error
+}
+
+type Message struct {
+	Content    string
+	Metadata   string
+	SeqNum     uint64
+	IsResponse bool
+	CreatedAt  int64
+}
+