Const-correctness for MD5Builder (#1175)

[comino]

• const for constant function arguments of MD5Builder
  solves : Const-correctness in MD5 and Hash library #1175

[codecov-io]

Codecov Report

Merging #3222 into master will decrease coverage by 0.02%.
The diff coverage is 88.88%.

@@            Coverage Diff             @@
##           master    #3222      +/-   ##
==========================================
- Coverage    27.6%   27.57%   -0.03%     
==========================================
  Files          20       20              
  Lines        3655     3655              
  Branches      678      678              
==========================================
- Hits         1009     1008       -1     
- Misses       2468     2469       +1     
  Partials      178      178

Impacted Files	Coverage Δ
cores/esp8266/MD5Builder.cpp	80.76% <88.88%> (ø)	⬆️
cores/esp8266/WString.h	62.5% <0%> (-12.5%)	⬇️

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