Remove custom trsen!

[blegat]

We have dropped Julia v0.7

[codecov-io]

Codecov Report

Merging #10 into master will increase coverage by 5.4%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master      #10     +/-   ##
=========================================
+ Coverage    82.6%   88.01%   +5.4%     
=========================================
  Files           9        9             
  Lines         391      367     -24     
=========================================
  Hits          323      323             
+ Misses         68       44     -24

Impacted Files	Coverage Δ
src/schur.jl	100% <100%> (+32%)	⬆️

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[coveralls]

Coverage increased (+5.8%) to 88.201% when pulling ca3b494 on bl/trsen! into 3e49d6c on master.