Add cbor_serialized_size

[PJK]

Description

• Prerequisite for fixing the repeated reallocation in cbor_serialize_alloc, which is super wasteful and ugly
• Adding as a part of the public API since it seems generally helpful
• Also add some new related test coverage

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 #250 (77ed96c) into master (0b084b9) will increase coverage by 0.47%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #250      +/-   ##
==========================================
+ Coverage   97.35%   97.83%   +0.47%     
==========================================
  Files          20       20              
  Lines        1589     1662      +73     
==========================================
+ Hits         1547     1626      +79     
+ Misses         42       36       -6     

Impacted Files	Coverage Δ
src/cbor/internal/memory_utils.c	95.83% <100.00%> (+2.08%)	⬆️
src/cbor/serialization.c	93.18% <100.00%> (+8.02%)	⬆️
src/cbor/streaming.c	99.00% <0.00%> (-1.00%)	⬇️

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