Resolve #1: Replace operations with messages

I've already removed operations from the system; in this commit I add a
new message type and integrate it into the transaction type.

libraries/chain/include/eos/chain/message.hpp

+#pragma once
+
+#include <eos/chain/protocol/types.hpp>
+
+namespace eos { namespace chain {
+
+/**
+ * @brief The message struct defines a blockchain message
+ *
+ * Messages are the heart of all activity on the blockchain -- all events and actions which take place in the chain are
+ * recorded as messages. Messages are sent from one account (@ref sender) to another account (@ref recipient), and are
+ * optionally also delivered to several other accounts (@ref notify_accounts).
+ *
+ * A message has a header that defines who sent it and who will be processing it. The message content is a binary blob,
+ * @ref data, whose type is determined by @ref type, which is dynamic and defined by the scripting language.
+ */
+struct message {
+   /// The account which sent the message
+   account sender;
+   /// The account to receive the message
+   account recipient;
+   /// Other accounts to notify about this message
+   vector<account> notify_accounts;
+   /// The message type -- this is defined by the contract(s) which create and/or process this message
+   message_type type;
+   /// The message contents
+   vector<char> data;
+};
+
+} } // namespace eos::chain
+
+FC_REFLECT(eos::chain::message, (sender)(recipient)(notify_accounts)(type)(data))

libraries/chain/include/eos/chain/protocol/transaction.hpp

@@ -23,6 +23,7 @@
  */
 #pragma once
 #include <eos/chain/protocol/types.hpp>
+#include <eos/chain/message.hpp>
 
 #include <numeric>
 
@@ -81,7 +82,8 @@ namespace eos { namespace chain {
        */
       fc::time_point_sec expiration;
 
-      vector<string> messages;
+      /// The messages in this transaction
+      vector<message> messages;
 
       /// Calculate the digest for a transaction
       digest_type         digest()const;

libraries/chain/include/eos/chain/protocol/types.hpp

@@ -90,8 +90,11 @@ namespace eos { namespace chain {
    using chainbase::allocator;
    using shared_string = boost::interprocess::basic_string<char, std::char_traits<char>, allocator<char>>;
 
-   typedef fc::ecc::private_key        private_key_type;
-   typedef fc::sha256 chain_id_type;
+   using private_key_type = fc::ecc::private_key;
+   using chain_id_type = fc::sha256;
+
+   using account = std::string;
+   using message_type = std::string;
 
    /**
     *  List all object types from all namespaces here so they can