Add support for RFC8132 (FETCH/PATCH) methods and errors

[chrysn]

Especially FETCH is crucial for OSCORE; the rest was added for completion (including Hop Limit Reached introduced in RFC8768).

(By the way, if you'd like to automate the process somewhat more, I'm keeping an up-to-date list in the coap-numbers crate. With some macro magic, or additional data if anything is needed there, it should be well possible to avoid manual processes in more than one place).

[martindisch]

Good stuff. Whether I'd want to pull in coap-numbers depends on how frequently there are new additions to CoAP, but as long as there's not too much going on I'm fine with how it is currently. If it gets too annoying or you think it would massively facilitate integration with other parts of your ecosystem, I'll gladly consider it. Thanks again!