add term_variables/2 and ground/1

[ichiban]

#144

[codecov-commenter]

Codecov Report

Merging #151 (2df0d41) into main (4e11a28) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #151      +/-   ##
==========================================
+ Coverage   93.78%   93.81%   +0.02%     
==========================================
  Files          18       18              
  Lines        5217     5236      +19     
==========================================
+ Hits         4893     4912      +19     
  Misses        274      274              
  Partials       50       50              

Impacted Files	Coverage Δ
engine/builtin.go	91.05% <100.00%> (+0.07%)	⬆️
interpreter.go	92.22% <100.00%> (+0.04%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4e11a28...2df0d41. Read the comment docs.