Add tests and eval.go

[tenntenn]

• Add tests
• Fix bugs
• Add eval.go

[codecov-io]

Codecov Report

Merging #8 into master will increase coverage by 38.75%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #8       +/-   ##
==========================================
+ Coverage   59.55%   98.3%   +38.75%     
==========================================
  Files           1       2        +1     
  Lines          89     118       +29     
==========================================
+ Hits           53     116       +63     
+ Misses         25       2       -23     
+ Partials       11       0       -11

Impacted Files	Coverage Δ
gpath.go	97.46% <100%> (+37.91%)	⬆️
eval.go	100% <100%> (ø)

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