invalidate code cache with references to virtual global variables on their update

[aviatesk]

closes #29

[codecov-commenter]

Codecov Report

Merging #30 into master will increase coverage by 0.55%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #30      +/-   ##
==========================================
+ Coverage   72.75%   73.30%   +0.55%     
==========================================
  Files          10       10              
  Lines         679      693      +14     
==========================================
+ Hits          494      508      +14     
  Misses        185      185              

Flag	Coverage Δ
#unittests	73.30% <100.00%> (+0.55%)	⬆️

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Impacted Files	Coverage Δ
src/TypeProfiler.jl	100.00% <ø> (ø)
src/abstractinterpreterinterface.jl	86.36% <ø> (ø)
src/abstractinterpretation.jl	98.34% <100.00%> (+0.18%)	⬆️
src/tfuncs.jl	94.44% <100.00%> (+0.69%)	⬆️

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