remove useless EXTENDED_ARG 0

[SnoopyLane]

This fixes #25

to_bytecode() now removes EXTENDED_ARG 0 instructions that appear in the input. These are useless, but can be left behind by ConcreteBytecode.from_code. Removing these from the input prevents analyzers from having to deal with EXTENDED_ARG instructions. Most of them are already removed, and having extras laying around causes failure when converting back to code.

I addition, Python 3.6' dis.stack_effect will raise ValueError when passed EXTENDED ARG, so anybody encountering these instructions must remove them if they wish to convert back to runnable code.

This commit adds a list deletions to ConcreteBytecode.to_bytecode() which tracks Instrs to be deleted in new later pass. Processing those deletions is proportional to the number of instructions to delete, not the number of instructions in the Bytecode. I.e. it should be fast for normal code that doesn't have these silly useless instructions.

[codecov-io]

Codecov Report

Merging #26 into master will decrease coverage by 0.02%. The diff coverage is 85.71%.

@@            Coverage Diff            @@
##           master     #26      +/-   ##
=========================================
- Coverage   93.22%   93.2%   -0.03%     
=========================================
  Files          17      17              
  Lines        2481    2488       +7     
=========================================
+ Hits         2313    2319       +6     
- Misses        168     169       +1

Impacted Files	Coverage Δ
bytecode/concrete.py	94.13% <85.71%> (-0.17%)	:arrow_down:

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[MatthieuDartiailh]

I do not believe this is the right fix see #28

[SnoopyLane]

Actually I like yours better.