Refactor contract code

[mtfishman]

Refactor contract code so that only contractadd! dispatches on the algorithm (otherwise it gets passed along as a keyword argument). Also rename output_labels(contract, ...) to contract_labels(...).

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 73.27%. Comparing base (1a718f1) to head (02b7739).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/contract/contractalgorithm.jl	78.57%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #114      +/-   ##
==========================================
- Coverage   73.96%   73.27%   -0.70%     
==========================================
  Files          20       21       +1     
  Lines        1091     1089       -2     
==========================================
- Hits          807      798       -9     
- Misses        284      291       +7     

Flag	Coverage Δ
docs	0.00% <0.00%> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.