Use array fragment for message body

[acogoluegnes]

Sometimes the message body can be just a subset of a byte array and the only way now to send this subset is to copy it into another array and use it as message body.

To avoid a copy, the Channel could be extended with basicPublish methods that accept an array fragment parameter (array of byte, offset, and length).

More context: #421, #421 (comment), #421 (comment).

[michaelklishin]

Something similar was brought up for .NET recently and was rejected due (but mostly due to questionable implementation choices and specific API changes).

[acogoluegnes]

Similar .NET feature mentioned by @michaelklishin: rabbitmq/rabbitmq-dotnet-client#489