Add ALU with test cases

[mlouielu]

No description provided.

[coveralls]

Coverage increased (+3.5%) to 96.319% when pulling 4e9bcf5 on impl_alu into 768624e on master.

[codecov-io]

Codecov Report

Merging #1 into master will increase coverage by 3.46%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   92.85%   96.31%   +3.46%     
==========================================
  Files          11       12       +1     
  Lines         140      163      +23     
==========================================
+ Hits          130      157      +27     
+ Misses         10        6       -4

Impacted Files	Coverage Δ
nand2vm/gate/const.py	100% <100%> (ø)
nand2vm/gate/alu.py	100% <100%> (+100%)	⬆️
nand2vm/gate/__init__.py	100% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+3.5%) to 96.319% when pulling fea585b on impl_alu into 768624e on master.

[coveralls]

Coverage increased (+3.5%) to 96.319% when pulling fea585b on impl_alu into 768624e on master.

[coveralls]

Coverage increased (+3.5%) to 96.319% when pulling fea585b on impl_alu into 768624e on master.

[coveralls]

Coverage increased (+3.5%) to 96.319% when pulling fea585b on impl_alu into 768624e on master.