Add _CBOR_UNUSED for MSVC, fix a few warnings

[PJK]

Re #161

[codecov-io]

Codecov Report

Merging #165 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #165      +/-   ##
==========================================
+ Coverage   95.53%   95.59%   +0.05%     
==========================================
  Files          47       47              
  Lines        3357     3357              
  Branches      190      190              
==========================================
+ Hits         3207     3209       +2     
+ Misses         97       96       -1     
+ Partials       53       52       -1     

Impacted Files	Coverage Δ
src/cbor/internal/loaders.c	96.96% <0.00%> (+6.06%)	⬆️

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