CoAP API revamp

Gcoap in its current form has maintainability issues due to its ties to nanocoap as discussed during the summit. <details><summary>Content of the summit's pad on the topic (because I don't trust these things to persist)</summary>CoAP API (re-)design Teaser Gcoap and nanocoap not only exist in parallel, they also share data structures like the pdu_t or the handler. Changes to Gcoap have accumulated Gcoap-specific fields in the pdu_t, but also make it hard to use Gcoap where even larger changes would be needed (han...</details>