Update Message structure to include "code" field

[eosio]

Update Message structure to include "code" field

[bytemaster]

struct Message
   sender      AccountName # testing
   code        AccountName
   recipients  AccountName[]
   type        FuncName
   data        Bytes

This is the proposed structure for the message, rather than having "notifier" we shall just count them all as recipients... the "code" will indicate the contract where the code should come from, the "recipients" will be where the data segments the code should operate on, and each invocation will have a "context" or "current recipient" which references the instance of the execution.

Think of "recipients" as the pixels and "code" as the shader and "context" as the "this" pointer or "current pixel/vertex" in the shader.