eigen for triangular

[jishnub]

This uses the triangular structure in the eigenvalue decomposition.

On master,

julia> U = UpperTriangularToeplitz(rand(1000));

julia> @btime eigvals($U);
  9.915 ms (13 allocations: 7.92 MiB)

julia> @btime eigen($U);
  90.046 ms (15 allocations: 16.29 MiB)

This PR

julia> @btime eigvals($U);
  12.535 ns (0 allocations: 0 bytes)

julia> @btime eigen($U);
  68.200 ms (12 allocations: 30.54 MiB)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3d47002) 95.25% compared to head (a921dcb) 95.61%.
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   95.25%   95.61%   +0.35%     
==========================================
  Files           9        9              
  Lines         948      957       +9     
==========================================
+ Hits          903      915      +12     
+ Misses         45       42       -3     

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[github-actions]

Pull Request Test Coverage Report for Build 7443271105

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 95.611%

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Totals
Change from base Build 7443258038:	0.02%
Covered Lines:	915
Relevant Lines:	957

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💛 - Coveralls