Add initial std feature to allow using std::Box rather than heapless::Box

[MathiasKoch]

Fixes #43

[codecov-commenter]

Codecov Report

Merging #44 (4ba49dd) into master (2320664) will decrease coverage by 0.06%.
The diff coverage is 14.28%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   39.62%   39.55%   -0.07%     
==========================================
  Files          13       13              
  Lines        1176     1178       +2     
  Branches      324      327       +3     
==========================================
  Hits          466      466              
- Misses        563      565       +2     
  Partials      147      147              

Impacted Files	Coverage Δ
mqttrust_core/src/eventloop.rs	37.08% <0.00%> (-0.36%)	⬇️
mqttrust_core/src/lib.rs	32.25% <0.00%> (ø)
mqttrust_core/src/state.rs	52.00% <100.00%> (ø)

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