Eigen for FillArrays

[jishnub]

Rebase #256 with only the FillArray part implemented this time. This makes eigvals O(1) and eigen O(N^2)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c7c34aa) 99.89% compared to head (42f7c9d) 99.89%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #330   +/-   ##
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  Coverage   99.89%   99.89%           
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  Files           8        8           
  Lines         920      944   +24     
=======================================
+ Hits          919      943   +24     
  Misses          1        1           

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[dlfivefifty]

This (and other parts that depend on LinearAlgebra.jl) should probably be an extension? Though I guess that can wait

[jishnub]

Yes, we should move these to an extension eventually. Presumably, LinearAlgebra will be excised in Julia v1.11, so we should do it in the next few months.