Add [[nodiscard]]-like macro and fix a few violations

[PJK]

Description

First step towards #247

Marking all functions where ignoring the return value

• is dangerous, or
• doesn't make sense since the function has no other effect (indicates incorrect usage)

Checklist

• I have read followed CONTRIBUTING.md
  • I have added tests
  • I have updated the documentation
  • I have updated the CHANGELOG
• Are there any breaking changes?
  • If yes: I have marked them in the CHANGELOG (example)
• Does this PR introduce any platform specific code?
• Security: Does this PR potentially affect security?
• Performance: Does this PR potentially affect performance?

[codecov-commenter]

Codecov Report

Merging #248 (f63418f) into master (c5e5731) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #248   +/-   ##
=======================================
  Coverage   97.26%   97.26%           
=======================================
  Files          20       20           
  Lines        1537     1537           
=======================================
  Hits         1495     1495           
  Misses         42       42           

Impacted Files	Coverage Δ
src/cbor.c	99.44% <ø> (ø)

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