Put COBS dependency and functions behind a feature flag

[bplaat]

Maybe I'm wrong, but I saw with a brief look at the code that the code that provided COBS encoding/decoding can be made optional. I use postcard in my project purely as a bin serialize data store, so it would be nice if *_cobs and the cobs dependency could be put behind a (default) feature flag.

@jamesmunns if you think this is a good idea I will make it and create a pr, but I wanted to hear your opinion first :^)

[jamesmunns]

I'm fairly certain this would be a breaking change. If a user has default features enabled today, and uses cobs, and we make it an optional feature, then postcard will break.

[bplaat]

Yes that's true, I had in my head that postcard was at a 0.x.x version, so breaking things was less dangerous, but it's not. Putting this future request for a 2.0 release is a great idea.