Implement a dumping AMQP values without using pn_data_t

Printing AMQP values is widely used in proton especially for frame tracing. It is massively inefficient because it relies on there being an existing pn_data_t structure which represents the AMQP value.
In the upcoming work to remove pn_data_t use from (nearly all of ) the critical path of proton-c there will no longer be such a pn_data_t that represents the received or transmitted frame. So we need new code to dump the AMQP values. Luckily the new AMQP consume code provides a simple skeleton which makes this fairly straightforward.
This new code can now produce text dumps of AMQP frames/values direct form the bytes encoding the frames with no intermediate step. This allows the use if this code in th elowest level send and receive code of the AMQP engine.