Clean up directory structure (#426)

* Remove unused circular-buffer crate
* Move transports into subdirectory
* Move misc into subdirectory
* Move stores into subdirectory
* Move multiplexers
* Move protocols
* Move libp2p top layer
* Fix Test: skip doctest if secio isn't enabled

protocols/identify/structs.proto

@@ -0,0 +1,23 @@
+message Identify {
+  // protocolVersion determines compatibility between peers
+  optional string protocolVersion = 5; // e.g. ipfs/1.0.0
+
+  // agentVersion is like a UserAgent string in browsers, or client version in bittorrent
+  // includes the client name and client.
+  optional string agentVersion = 6; // e.g. go-ipfs/0.1.0
+
+  // publicKey is this node's public key (which also gives its node.ID)
+  // - may not need to be sent, as secure channel implies it has been sent.
+  // - then again, if we change / disable secure channel, may still want it.
+  optional bytes publicKey = 1;
+
+  // listenAddrs are the multiaddrs the sender node listens for open connections on
+  repeated bytes listenAddrs = 2;
+
+  // oservedAddr is the multiaddr of the remote endpoint that the sender node perceives
+  // this is useful information to convey to the other side, as it helps the remote endpoint
+  // determine whether its connection to the local peer goes through NAT.
+  optional bytes observedAddr = 4;
+
+  repeated string protocols = 3;
+}