Introduce client/server separation

Refactor to introduce client/server separation, including a typed
client API and a HTTP REST server implementation.

Refactor the terminal to be an API consumer.

terminal/command_test.go

@@ -0,0 +1,53 @@
+package terminal
+
+import (
+	"fmt"
+	"testing"
+
+	"github.com/derekparker/delve/service"
+)
+
+func TestCommandDefault(t *testing.T) {
+	var (
+		cmds = Commands{}
+		cmd  = cmds.Find("non-existant-command")
+	)
+
+	err := cmd(nil)
+	if err == nil {
+		t.Fatal("cmd() did not default")
+	}
+
+	if err.Error() != "command not available" {
+		t.Fatal("wrong command output")
+	}
+}
+
+func TestCommandReplay(t *testing.T) {
+	cmds := DebugCommands(nil)
+	cmds.Register("foo", func(client service.Client, args ...string) error { return fmt.Errorf("registered command") }, "foo command")
+	cmd := cmds.Find("foo")
+
+	err := cmd(nil)
+	if err.Error() != "registered command" {
+		t.Fatal("wrong command output")
+	}
+
+	cmd = cmds.Find("")
+	err = cmd(nil)
+	if err.Error() != "registered command" {
+		t.Fatal("wrong command output")
+	}
+}
+
+func TestCommandReplayWithoutPreviousCommand(t *testing.T) {
+	var (
+		cmds = DebugCommands(nil)
+		cmd  = cmds.Find("")
+		err  = cmd(nil)
+	)
+
+	if err != nil {
+		t.Error("Null command not returned", err)
+	}
+}