This changes the API of nanocoap with the goal to reduce the expose of
UDP specifics in the API. The plan is to eventually support transports
such as CoAP over TCP and CoAP over WebSocket directly in nanocoap
while sharing most of the code, as e.g. the CoAP Option processing
remains identical. Specifically, the plan is to unlock a transport with
modules and introduce overhead for dispatching to specific transport
only when multiple transports are actually in use.

Support for OSCORE directly in nanocoap is probably not sensible, as
the serialization is very much unlike the other transports. A unified
CoAP API for multiple transports including OSCORE is probably best
implemented on top. But when limited to the boring set of CoAP
transports, we probably can support them well with nanocoap with less
overhead.

Breaking API Changes:
=====================

- `coap_parse()` now returns `ssize_t` instead of `int`
    - This function is not really user facing, so the impact should
      be limited
    - This is useful for stream transports where the buffer may
      contain data of more than one packet. The return value contains
      the number of bytes actually consumed, which will match the
      buffer size for non-stream transports.

API Changes:
============

- `coap_pkt_t` now contains a `uint8_t *buf` pointer instead of a
  `coap_hdr_t *hdr` pointer to the beginning of the buffer
    - This will also work when the buffer is used by non-UDP
      transports
    - A deprecated `coap_udp_hdr_t *hdr` has been crammed into
      an unnamed `union` with `uint8_t *buf`. For architectures
      where pointers have the same memory layout regardless of type
      (e.g. all of the supported ones), this will make `hdr` an
      alias for `buf`.
    - The alias will only be provided if no transport besides UDP is
      used in nanocoap. So existing apps will continue to work, new
      apps that want to support other transports need to move to
      adapt.
- `coap_hdr_t` has been renamed to `coap_udp_hdr_t`
    - A deprecated alias was created for deprecation
- `coap_hdr*()` functions have been deprecated
    - Equivalent `coap_pkt*()` functions have been created that work
      on `coap_pkt_t *` instead of `coap_hdr_t *`
    - If non-UDP transports are used, the deprecated `coap_hdr*()`
      will probably not be exposed to avoid footguns.
- `coap_build_hdr()` has been renamed to `coap_build_udp_hdr()` and
  that works on an `uint8_t *` buffer with a given length, rather than
  on a `coap_hdr_t *` with a *figers crossed* length
    - a deprecated `coap_build_hdr()` function was added that calls
      to `coap_build_udp_hdr()` and has the same signature, so that
      users have time to update