Improve cbor_stream_decode interface

[PJK]

• Remove CBOR_DECODER_EBUFFER as it was never used and provides no benefit over NEDATA
• Change cbor_decoder_result.required to correctly account for all the data needed to proceed
• Add test coverage
• Improve comments & documentation
• Minor code cleanup

Closes #106

[codecov-commenter]

Codecov Report

Merging #156 into master will increase coverage by 4.26%.
The diff coverage is 88.09%.

@@            Coverage Diff             @@
##           master     #156      +/-   ##
==========================================
+ Coverage   88.43%   92.69%   +4.26%     
==========================================
  Files          53       52       -1     
  Lines        3561     3452     -109     
  Branches      198      199       +1     
==========================================
+ Hits         3149     3200      +51     
+ Misses        332      198     -134     
+ Partials       80       54      -26     

Impacted Files	Coverage Δ
src/cbor.c	97.83% <ø> (ø)
test/assertions.c	0.00% <0.00%> (ø)
src/cbor/streaming.c	96.29% <84.78%> (+11.11%)	⬆️
test/cbor_stream_decode_test.c	100.00% <100.00%> (ø)

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