istril/istriu for opposite triangularity

[jishnub]

This adds specializations for istril(::UpperTriangular) and istriu(::LowerTriangular), and similarly for the unit triangular matrices. By forwarding the operations to the parent, we may improve performance:

julia> Z = zeros(400,400);

julia> @btime istril(UpperTriangular($Z));
  115.105 μs (0 allocations: 0 bytes) # master
  20.188 μs (0 allocations: 0 bytes) # this PR

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.97%. Comparing base (925acef) to head (9675c38).
Report is 9 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1265      +/-   ##
==========================================
- Coverage   92.03%   91.97%   -0.07%     
==========================================
  Files          34       34              
  Lines       15475    15479       +4     
==========================================
- Hits        14243    14237       -6     
- Misses       1232     1242      +10     

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