Update for constraint objects

[wilfwilson]

This makes GraphBacktracking work with peal/BacktrackKit#21

[codecov]

Codecov Report

Merging #4 (3912777) into master (34e7351) will decrease coverage by 0.09%.
The diff coverage is 100.00%.

❗ Current head 3912777 differs from pull request most recent head c73df32. Consider uploading reports for the commit c73df32 to get more accurate results

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
- Coverage   95.98%   95.89%   -0.10%     
==========================================
  Files           8        8              
  Lines         498      487      -11     
==========================================
- Hits          478      467      -11     
  Misses         20       20              

Impacted Files	Coverage Δ
gap/constraints/canonicalconstraints.g	100.00% <100.00%> (ø)
gap/constraints/conjugacy.g	100.00% <100.00%> (ø)
gap/constraints/normaliser.g	100.00% <100.00%> (ø)
gap/constraints/simpleconstraints.g	97.72% <100.00%> (-0.08%)	⬇️