async-coap-uri: Rewrite AnyUriRef::write_resolved to not depend on allocation

[darconeous]

AnyUriRef::write_resolved, as currently written, depends on the std crate, due to the use of dynamically-sized container types. It should be possible to rewrite this method in such a way that this dependency is no longer required.

This issue is the largest impediment to allowing async-coap-uri to work in no-std environments.

Additionally, while the method does currently pass the rather large unit test corpus, the implementation is a bit of a mess. Any re-write should also attempt to make the code easier to follow.