Add a basic IPC setup with Channel Object.

Pass a process a channel endpoint on startup that it will use to
get it's initial capabilities.

zion/object/channel.h

@@ -0,0 +1,40 @@
+#pragma once
+
+#include "capability/capability.h"
+#include "lib/linked_list.h"
+#include "lib/pair.h"
+#include "lib/ref_ptr.h"
+#include "object/kernel_object.h"
+#include "usr/zcall_internal.h"
+
+class Channel : public KernelObject {
+ public:
+  static Pair<RefPtr<Channel>, RefPtr<Channel>> CreateChannelPair();
+
+  RefPtr<Channel> peer() { return peer_; }
+
+  uint64_t Write(const ZMessage& msg);
+  uint64_t Read(ZMessage& msg);
+
+ private:
+  // FIXME: We will likely never close the channel based on this
+  // circular dependency.
+  RefPtr<Channel> peer_{nullptr};
+
+  struct Message {
+    uint64_t type;
+
+    uint64_t num_bytes;
+    uint8_t* bytes;
+  };
+
+  // FIXME: This is probably dangerous because of an
+  // implicit shallow copy.
+  LinkedList<Message> pending_messages_;
+
+  friend class MakeRefCountedFriend<Channel>;
+  Channel() {}
+  void SetPeer(const RefPtr<Channel>& peer) { peer_ = peer; }
+
+  uint64_t EnqueueMessage(const ZMessage& msg);
+};