Implement ExtractFromBinary and InjectToBinary traits

[matevz]

I implemented basic ExtractFromBinary and InjectToBinary traits:

• Extracts and injects trace_id, span_id, parent_span_id, and flags.
• Tested with https://github.com/jaegertracing/jaeger-client-go
• No support for baggage items yet

Please review and merge.

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 6.37%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   60.34%   66.72%   +6.37%     
==========================================
  Files           7        7              
  Lines         517      598      +81     
==========================================
+ Hits          312      399      +87     
+ Misses        205      199       -6

Impacted Files	Coverage Δ
src/span.rs	82.99% <100%> (+8.79%)	⬆️
src/tracer.rs	77.27% <0%> (+5.05%)	⬆️

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[sile]

Thank you for your PR. I reviewed it and have left some comments. Please check them.

[matevz]

Thanks for the review. I addressed the issues.

[sile]

LGTM! Thanks for your contribution.