flags: fix the flag inference mechanism

[MatthieuDartiailh]

The existing implementation could easily generate invalid flags and was not easy to use to forcibly set a function as asynchronous or not. The new implementation address both limitations.

[codecov-io]

Codecov Report

Merging #56 into master will increase coverage by 3.89%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #56      +/-   ##
==========================================
+ Coverage   94.27%   98.16%   +3.89%     
==========================================
  Files          17        7      -10     
  Lines        2847     1420    -1427     
==========================================
- Hits         2684     1394    -1290     
+ Misses        163       26     -137

Impacted Files	Coverage Δ
bytecode/bytecode.py	99.33% <100%> (+7.61%)	⬆️
bytecode/flags.py	100% <100%> (ø)	⬆️
bytecode/peephole_opt.py	94.38% <0%> (-0.02%)	⬇️
bytecode/tests/__init__.py
setup.py
bytecode/concrete.py	97.91% <0%> (+1.15%)	⬆️
bytecode/__init__.py	100% <0%> (+1.19%)	⬆️
bytecode/cfg.py	100% <0%> (+3.07%)	⬆️
bytecode/instr.py	99.48% <0%> (+5.18%)	⬆️

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