incremental decode

[charlesnicholson]

Implements #48 by introducing cobs_decode_inc_begin and cobs_decode_inc. Improvements over cobs_encode_inc interface by allowing each separate call to cobs_decode_inc to take new src/dst pointers, which allows the caller to not have to store the entire encoded/decoded buffer. Useful for low-memory streaming environments where you want to decode-and-stream incrementally.

It also overhauls the CI + sanitizer stuff: sanitizers are run as separate jobs because they're not all compatible. All jobs now flow into an all-checks-passed job so I don't have to keep updating the github branch protection pages.

Also some C++20 modernization over in the tests.