Add support for coap-message-implementations 0.2

coap-message-implementations went through a breaking change; this crate uses it as part of its public API, so the PR compatibly adds the newer version through a feature (and sets the older version as a default feature).

Part of that breaking change was not silently truncating payload any more, expecting the callers to be explicit in when they don't care about previously mapped or set payload any more. This is sensible, but does not match how libOSCORE handles things.

The workaround is crude and described / tracked in #63.