Messages: improve support

[intigonzalez]

The current status of messages is this:

• Partially added to the syntax: you can use fields when you are sending a message
• You cannot declare fields beforehand
• You cannot use them on guards

We must add the missing features to:

1. The language (Floreal can use whatever syntax he is happy with)
2. The compiler, I will go for a simple representation of messages (all messages are represented by the same structure in the target environment, kind of key-value store)
3. Runtime support to marshal/unmarshal messages, to send them and to access their fields