Ditch libraries

[ichiban]

When I introduced DCG to this library, its term expansion slowed everything down so I pushed it into an optional library.
I don't see any use cases for libraries since this is just a Go package.

• Reimplement DCG in Go so that it doesn't slow down
• Remove libraries

[codecov-commenter]

Codecov Report

Merging #146 (3051fea) into main (3b914d7) will increase coverage by 0.29%.
The diff coverage is 97.39%.

@@            Coverage Diff             @@
##             main     #146      +/-   ##
==========================================
+ Coverage   93.47%   93.77%   +0.29%     
==========================================
  Files          18       18              
  Lines        5092     5205     +113     
==========================================
+ Hits         4760     4881     +121     
+ Misses        284      274      -10     
- Partials       48       50       +2     

Impacted Files	Coverage Δ
engine/dcg.go	95.86% <95.86%> (ø)
engine/builtin.go	90.95% <100.00%> (+0.05%)	⬆️
engine/clause.go	97.82% <100.00%> (+9.49%)	⬆️
engine/compound.go	100.00% <100.00%> (ø)
engine/vm.go	80.78% <100.00%> (+0.12%)	⬆️
interpreter.go	91.93% <100.00%> (-2.60%)	⬇️

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