[Rust] Implement Display for encoders and decoders

[marciorasf]

When using sbe-tools to generate java code, the encoders and decoders implement toString() that returns a human-readable representation of all the fields of the message. For example:

[Sequence](sbeTemplateId=9|sbeSchemaId=1|sbeSchemaVersion=3|sbeBlockLength=4):messageType=Sequence|nextSeqNo=629

Having a human-readable representation makes debugging much, much easier, as you can print the SBE messages and check what exactly you're sending/receiving.

When generating rust, currently there's nothing like the java's toString(). The encoders and decoders derive Debug, but it prints just the name of the struct and the buffer, which is not that useful.

Could we implement Display using the exactly same format as Java for both encoders and decoders?

[marciorasf]

Update: I'm working on it and I expect to submit a proposal soon.