Add message converters

[asdine]

This PR introduces the message formatters and unformatters to Felice.

Producer

• New MessageConverter interface
• MessageConverterV1 is the default implementation of the MessageConverter
• Selecting a MessageConverter is mandatory when creating a producer

Consumer

• New MessageConverter interface
• MessageConverterV1 is the default implementation of the MessageConverter
• Selecting a MessageConverter is mandatory when calling setting a handler per topic

Other changes

• The message package has been removed. It was difficult to have one type for both the producer and consumer while keeping this type coherent. I created one Message for both packages which is specialized for use in its package.
• The handlers had been removed and its code was integrated in the consumer package. With the removal of the message package, we had an import cycle between the consumer package and the handlers package, which forced me to reconsider the organization of this package.
  I think for now a Handler is strongly coupled with a Consumer and it is normal to have them in the same package. I also unexported the Collection type as I don't yet see how it can be useful to be used outside of this package. Let me know if you disagree

[tealeg]

🌷 The "checks if" at the start of all these test descriptions is redundant. We know they're checking something. It's better to just make a plain statement. Also, this sort of case might not even need a comment at all (I often write them also, but I don't think "it does the right thing" is adding anything much).

[asdine]

🆙

[asdine]

🆙 @tealeg

[tealeg]

👍 Looks good - I've made suggestions for a couple of minor fixes in the English in the doc.go, but otherwise excellent, and you can merge when they're applied.