Fix length

[ichiban]

#191

This PR re-implements length/2 as described in http://www.complang.tuwien.ac.at/ulrich/iso-prolog/prologue#length

Also, improved cycle detection with Brent's cycle detection algorithm.

[codecov-commenter]

Codecov Report

Merging #217 (09049e9) into main (c84f442) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #217      +/-   ##
==========================================
+ Coverage   97.13%   97.18%   +0.04%     
==========================================
  Files          21       21              
  Lines        6394     6505     +111     
==========================================
+ Hits         6211     6322     +111     
  Misses        151      151              
  Partials       32       32              

Impacted Files	Coverage Δ
cmd/1pl/interpreter.go	100.00% <100.00%> (ø)
engine/builtin.go	96.58% <100.00%> (+0.13%)	⬆️
engine/compound.go	100.00% <100.00%> (ø)
engine/env.go	98.26% <100.00%> (+0.09%)	⬆️
engine/iterator.go	100.00% <100.00%> (ø)
engine/number.go	100.00% <100.00%> (ø)
engine/promise.go	100.00% <100.00%> (ø)
interpreter.go	94.90% <100.00%> (+0.02%)	⬆️

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