Implement compacted hessian_lagrangian

[odow]

Closes #13

@ccoffrin this was what I was talking about.

[codecov-commenter]

Codecov Report

Merging #14 (3c181a6) into master (26241fa) will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #14      +/-   ##
==========================================
+ Coverage   97.92%   97.98%   +0.06%     
==========================================
  Files           3        3              
  Lines         433      446      +13     
==========================================
+ Hits          424      437      +13     
  Misses          9        9              

Impacted Files	Coverage Δ
src/nonlinear_oracle.jl	97.73% <100.00%> (+0.09%)	⬆️

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[odow]

Can you try a new benchmark run on the latest master? I'm seeing massively reduced hessian counts, so it'd be interesting to see if this is the difference.