Refactor debug logging

[etingof]

Debug logging refactored for more efficiency when disabled and for more functionality when in use.

Specifically, the global LOG object can easily be used from any function/method, not just from codec main loop as it used to be.

[codecov-io]

Codecov Report

Merging #138 into master will increase coverage by 0.07%.
The diff coverage is 88.31%.

@@            Coverage Diff             @@
##           master     #138      +/-   ##
==========================================
+ Coverage   85.67%   85.75%   +0.07%     
==========================================
  Files          29       29              
  Lines        3987     3987              
==========================================
+ Hits         3416     3419       +3     
+ Misses        571      568       -3

Impacted Files	Coverage Δ
pyasn1/codec/native/decoder.py	84.84% <100%> (+1.02%)	⬆️
pyasn1/codec/native/encoder.py	87.91% <100%> (+0.81%)	⬆️
pyasn1/codec/ber/encoder.py	90.15% <77.77%> (+0.2%)	⬆️
pyasn1/codec/ber/decoder.py	86.94% <82.85%> (-0.04%)	⬇️
pyasn1/debug.py	94.11% <94.73%> (+0.61%)	⬆️

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