Refactoring struct.unpack and struct.pack usage

[nlachfr]

I've put all the packing / unpacking work in a class DalvikPacker which is initialized in ClassManager during header parsing.

I had to rewrite some functions defined in the top of androguard.core.bytecodes.dvm package in order to access the class manager.

Resolves : #644

[codecov-io]

Codecov Report

Merging #693 into master will increase coverage by 0.1%.
The diff coverage is 68.1%.

@@            Coverage Diff            @@
##           master     #693     +/-   ##
=========================================
+ Coverage    72.6%   72.71%   +0.1%     
=========================================
  Files          50       50             
  Lines       16188    16135     -53     
=========================================
- Hits        11754    11733     -21     
+ Misses       4434     4402     -32

Impacted Files	Coverage Δ
tests/parse_dex.py	97.89% <100%> (+0.11%)	⬆️
tests/test_dex.py	98.36% <100%> (+0.11%)	⬆️
androguard/core/bytecodes/dvm.py	63.01% <65.2%> (+0.19%)	⬆️
androguard/core/bytecode.py	43.87% <0%> (+0.24%)	⬆️

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