Migrate to embedded-time

[MathiasKoch]

Fixes #56

[codecov]

Codecov Report

Merging #77 (a081873) into master (da7f28c) will increase coverage by 0.25%.
The diff coverage is 37.20%.

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   33.48%   33.74%   +0.25%     
==========================================
  Files          20       20              
  Lines        1117     1144      +27     
==========================================
+ Hits          374      386      +12     
- Misses        743      758      +15     

Impacted Files	Coverage Δ
atat/src/lib.rs	6.25% <0.00%> (-0.21%)	⬇️
atat/src/traits.rs	31.25% <0.00%> (-10.42%)	⬇️
atat/src/client.rs	50.00% <44.11%> (+1.56%)	⬆️
atat/src/ingress_manager.rs	52.70% <100.00%> (ø)

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[MathiasKoch]

Closed in favor of #107